diff --git a/apps/web/src/app/(authenticated)/settings/domains/page.tsx b/apps/web/src/app/(authenticated)/settings/domains/page.tsx
index a945f87..20ca4a7 100644
--- a/apps/web/src/app/(authenticated)/settings/domains/page.tsx
+++ b/apps/web/src/app/(authenticated)/settings/domains/page.tsx
@@ -1,23 +1,383 @@
 "use client";
 
-import { useState, useEffect } from "react";
+import { useState, useEffect, type SyntheticEvent } from "react";
+import type { ReactElement } from "react";
 import type { Domain } from "@mosaic/shared";
 import { DomainList } from "@/components/domains/DomainList";
-import { fetchDomains, deleteDomain } from "@/lib/api/domains";
+import { fetchDomains, createDomain, deleteDomain } from "@/lib/api/domains";
+import type { CreateDomainDto } from "@/lib/api/domains";
+import { useWorkspaceId } from "@/lib/hooks";
 
-export default function DomainsPage(): React.ReactElement {
+/* ---------------------------------------------------------------------------
+   Slug generation helper
+   --------------------------------------------------------------------------- */
+
+function generateSlug(name: string): string {
+  return name
+    .toLowerCase()
+    .trim()
+    .replace(/[^a-z0-9\s-]/g, "")
+    .replace(/\s+/g, "-")
+    .replace(/-+/g, "-")
+    .slice(0, 100);
+}
+
+/* ---------------------------------------------------------------------------
+   Create Domain Dialog
+   --------------------------------------------------------------------------- */
+
+interface CreateDomainDialogProps {
+  open: boolean;
+  onOpenChange: (open: boolean) => void;
+  onSubmit: (data: CreateDomainDto) => Promise<void>;
+  isSubmitting: boolean;
+}
+
+function CreateDomainDialog({
+  open,
+  onOpenChange,
+  onSubmit,
+  isSubmitting,
+}: CreateDomainDialogProps): ReactElement | null {
+  const [name, setName] = useState("");
+  const [slug, setSlug] = useState("");
+  const [slugTouched, setSlugTouched] = useState(false);
+  const [description, setDescription] = useState("");
+  const [formError, setFormError] = useState<string | null>(null);
+
+  function resetForm(): void {
+    setName("");
+    setSlug("");
+    setSlugTouched(false);
+    setDescription("");
+    setFormError(null);
+  }
+
+  function handleNameChange(value: string): void {
+    setName(value);
+    if (!slugTouched) {
+      setSlug(generateSlug(value));
+    }
+  }
+
+  function handleSlugChange(value: string): void {
+    setSlugTouched(true);
+    setSlug(value.toLowerCase().replace(/[^a-z0-9-]/g, ""));
+  }
+
+  async function handleSubmit(e: SyntheticEvent): Promise<void> {
+    e.preventDefault();
+    setFormError(null);
+
+    const trimmedName = name.trim();
+    if (!trimmedName) {
+      setFormError("Domain name is required.");
+      return;
+    }
+
+    const trimmedSlug = slug.trim();
+    if (!trimmedSlug) {
+      setFormError("Slug is required.");
+      return;
+    }
+
+    if (!/^[a-z0-9-]+$/.test(trimmedSlug)) {
+      setFormError("Slug must contain only lowercase letters, numbers, and hyphens.");
+      return;
+    }
+
+    try {
+      const payload: CreateDomainDto = { name: trimmedName, slug: trimmedSlug };
+      const trimmedDesc = description.trim();
+      if (trimmedDesc) {
+        payload.description = trimmedDesc;
+      }
+      await onSubmit(payload);
+      resetForm();
+    } catch (err: unknown) {
+      setFormError(err instanceof Error ? err.message : "Failed to create domain.");
+    }
+  }
+
+  if (!open) return null;
+
+  return (
+    <div
+      role="dialog"
+      aria-modal="true"
+      aria-labelledby="create-domain-title"
+      style={{
+        position: "fixed",
+        inset: 0,
+        zIndex: 50,
+        display: "flex",
+        alignItems: "center",
+        justifyContent: "center",
+      }}
+    >
+      {/* Backdrop */}
+      <div
+        style={{
+          position: "fixed",
+          inset: 0,
+          background: "rgba(0,0,0,0.5)",
+        }}
+        onClick={() => {
+          if (!isSubmitting) {
+            resetForm();
+            onOpenChange(false);
+          }
+        }}
+      />
+
+      {/* Dialog */}
+      <div
+        style={{
+          position: "relative",
+          background: "var(--surface, #fff)",
+          borderRadius: "8px",
+          border: "1px solid var(--border, #e5e7eb)",
+          padding: 24,
+          width: "100%",
+          maxWidth: 480,
+          zIndex: 1,
+        }}
+      >
+        <h2
+          id="create-domain-title"
+          style={{
+            fontSize: "1.125rem",
+            fontWeight: 600,
+            color: "var(--text, #111)",
+            margin: "0 0 8px",
+          }}
+        >
+          New Domain
+        </h2>
+        <p style={{ color: "var(--muted, #6b7280)", fontSize: "0.875rem", margin: "0 0 16px" }}>
+          Domains help you organize tasks, projects, and events by life area.
+        </p>
+
+        <form
+          onSubmit={(e) => {
+            void handleSubmit(e);
+          }}
+        >
+          {/* Name */}
+          <div style={{ marginBottom: 16 }}>
+            <label
+              htmlFor="domain-name"
+              style={{
+                display: "block",
+                marginBottom: 6,
+                fontSize: "0.85rem",
+                fontWeight: 500,
+                color: "var(--text-2, #374151)",
+              }}
+            >
+              Name <span style={{ color: "var(--danger, #ef4444)" }}>*</span>
+            </label>
+            <input
+              id="domain-name"
+              type="text"
+              value={name}
+              onChange={(e) => {
+                handleNameChange(e.target.value);
+              }}
+              placeholder="e.g. Personal Finance"
+              maxLength={255}
+              autoFocus
+              style={{
+                width: "100%",
+                padding: "8px 12px",
+                background: "var(--bg, #f9fafb)",
+                border: "1px solid var(--border, #d1d5db)",
+                borderRadius: "6px",
+                color: "var(--text, #111)",
+                fontSize: "0.9rem",
+                outline: "none",
+                boxSizing: "border-box",
+              }}
+            />
+          </div>
+
+          {/* Slug */}
+          <div style={{ marginBottom: 16 }}>
+            <label
+              htmlFor="domain-slug"
+              style={{
+                display: "block",
+                marginBottom: 6,
+                fontSize: "0.85rem",
+                fontWeight: 500,
+                color: "var(--text-2, #374151)",
+              }}
+            >
+              Slug <span style={{ color: "var(--danger, #ef4444)" }}>*</span>
+            </label>
+            <input
+              id="domain-slug"
+              type="text"
+              value={slug}
+              onChange={(e) => {
+                handleSlugChange(e.target.value);
+              }}
+              placeholder="e.g. personal-finance"
+              maxLength={100}
+              style={{
+                width: "100%",
+                padding: "8px 12px",
+                background: "var(--bg, #f9fafb)",
+                border: "1px solid var(--border, #d1d5db)",
+                borderRadius: "6px",
+                color: "var(--text, #111)",
+                fontSize: "0.9rem",
+                outline: "none",
+                boxSizing: "border-box",
+                fontFamily: "var(--mono, monospace)",
+              }}
+            />
+            <p
+              style={{
+                fontSize: "0.75rem",
+                color: "var(--muted, #6b7280)",
+                margin: "4px 0 0",
+              }}
+            >
+              Lowercase letters, numbers, and hyphens only.
+            </p>
+          </div>
+
+          {/* Description */}
+          <div style={{ marginBottom: 16 }}>
+            <label
+              htmlFor="domain-description"
+              style={{
+                display: "block",
+                marginBottom: 6,
+                fontSize: "0.85rem",
+                fontWeight: 500,
+                color: "var(--text-2, #374151)",
+              }}
+            >
+              Description
+            </label>
+            <textarea
+              id="domain-description"
+              value={description}
+              onChange={(e) => {
+                setDescription(e.target.value);
+              }}
+              placeholder="A brief summary of this domain..."
+              rows={3}
+              maxLength={10000}
+              style={{
+                width: "100%",
+                padding: "8px 12px",
+                background: "var(--bg, #f9fafb)",
+                border: "1px solid var(--border, #d1d5db)",
+                borderRadius: "6px",
+                color: "var(--text, #111)",
+                fontSize: "0.9rem",
+                outline: "none",
+                resize: "vertical",
+                fontFamily: "inherit",
+                boxSizing: "border-box",
+              }}
+            />
+          </div>
+
+          {/* Form error */}
+          {formError !== null && (
+            <p
+              style={{
+                color: "var(--danger, #ef4444)",
+                fontSize: "0.85rem",
+                margin: "0 0 12px",
+              }}
+            >
+              {formError}
+            </p>
+          )}
+
+          {/* Buttons */}
+          <div
+            style={{
+              display: "flex",
+              justifyContent: "flex-end",
+              gap: 8,
+              marginTop: 8,
+            }}
+          >
+            <button
+              type="button"
+              onClick={() => {
+                resetForm();
+                onOpenChange(false);
+              }}
+              disabled={isSubmitting}
+              style={{
+                padding: "8px 16px",
+                background: "transparent",
+                border: "1px solid var(--border, #d1d5db)",
+                borderRadius: "6px",
+                color: "var(--text-2, #374151)",
+                fontSize: "0.85rem",
+                cursor: "pointer",
+              }}
+            >
+              Cancel
+            </button>
+            <button
+              type="submit"
+              disabled={isSubmitting || !name.trim() || !slug.trim()}
+              style={{
+                padding: "8px 16px",
+                background: "var(--primary, #111827)",
+                border: "none",
+                borderRadius: "6px",
+                color: "#fff",
+                fontSize: "0.85rem",
+                fontWeight: 500,
+                cursor: isSubmitting || !name.trim() || !slug.trim() ? "not-allowed" : "pointer",
+                opacity: isSubmitting || !name.trim() || !slug.trim() ? 0.6 : 1,
+              }}
+            >
+              {isSubmitting ? "Creating..." : "Create Domain"}
+            </button>
+          </div>
+        </form>
+      </div>
+    </div>
+  );
+}
+
+/* ---------------------------------------------------------------------------
+   Domains Page
+   --------------------------------------------------------------------------- */
+
+export default function DomainsPage(): ReactElement {
+  const workspaceId = useWorkspaceId();
   const [domains, setDomains] = useState<Domain[]>([]);
   const [isLoading, setIsLoading] = useState<boolean>(true);
   const [error, setError] = useState<string | null>(null);
 
+  // Create dialog state
+  const [createOpen, setCreateOpen] = useState(false);
+  const [isCreating, setIsCreating] = useState(false);
+
   useEffect(() => {
+    if (!workspaceId) {
+      setIsLoading(false);
+      return;
+    }
     void loadDomains();
-  }, []);
+  }, [workspaceId]);
 
   async function loadDomains(): Promise<void> {
     try {
       setIsLoading(true);
-      const response = await fetchDomains();
+      const response = await fetchDomains(undefined, workspaceId ?? undefined);
       setDomains(response.data);
       setError(null);
     } catch (err) {
@@ -27,9 +387,8 @@ export default function DomainsPage(): React.ReactElement {
     }
   }
 
-  function handleEdit(domain: Domain): void {
+  function handleEdit(_domain: Domain): void {
     // TODO: Open edit modal/form
-    console.log("Edit domain:", domain);
   }
 
   async function handleDelete(domain: Domain): Promise<void> {
@@ -38,13 +397,26 @@ export default function DomainsPage(): React.ReactElement {
     }
 
     try {
-      await deleteDomain(domain.id);
+      await deleteDomain(domain.id, workspaceId ?? undefined);
       await loadDomains();
     } catch (err) {
       setError(err instanceof Error ? err.message : "Failed to delete domain");
     }
   }
 
+  async function handleCreate(data: CreateDomainDto): Promise<void> {
+    setIsCreating(true);
+    try {
+      await createDomain(data, workspaceId ?? undefined);
+      setCreateOpen(false);
+      await loadDomains();
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Failed to create domain.");
+    } finally {
+      setIsCreating(false);
+    }
+  }
+
   return (
     <div className="max-w-6xl mx-auto p-6">
       <div className="mb-6">
@@ -60,7 +432,7 @@ export default function DomainsPage(): React.ReactElement {
         <button
           className="px-4 py-2 bg-gray-900 text-white rounded hover:bg-gray-800"
           onClick={() => {
-            console.log("TODO: Open create modal");
+            setCreateOpen(true);
           }}
         >
           Create Domain
@@ -73,6 +445,13 @@ export default function DomainsPage(): React.ReactElement {
         onEdit={handleEdit}
         onDelete={handleDelete}
       />
+
+      <CreateDomainDialog
+        open={createOpen}
+        onOpenChange={setCreateOpen}
+        onSubmit={handleCreate}
+        isSubmitting={isCreating}
+      />
     </div>
   );
 }
diff --git a/apps/web/src/lib/api/domains.ts b/apps/web/src/lib/api/domains.ts
index 741bc1b..256f0a3 100644
--- a/apps/web/src/lib/api/domains.ts
+++ b/apps/web/src/lib/api/domains.ts
@@ -44,7 +44,10 @@ export interface DomainFilters {
 /**
  * Fetch all domains
  */
-export async function fetchDomains(filters?: DomainFilters): Promise<ApiResponse<Domain[]>> {
+export async function fetchDomains(
+  filters?: DomainFilters,
+  workspaceId?: string
+): Promise<ApiResponse<Domain[]>> {
   const params = new URLSearchParams();
 
   if (filters?.search) {
@@ -60,7 +63,7 @@ export async function fetchDomains(filters?: DomainFilters): Promise<ApiResponse
   const queryString = params.toString();
   const endpoint = queryString ? `/api/domains?${queryString}` : "/api/domains";
 
-  return apiGet<ApiResponse<Domain[]>>(endpoint);
+  return apiGet<ApiResponse<Domain[]>>(endpoint, workspaceId);
 }
 
 /**
@@ -73,20 +76,27 @@ export async function fetchDomain(id: string): Promise<DomainWithCounts> {
 /**
  * Create a new domain
  */
-export async function createDomain(data: CreateDomainDto): Promise<Domain> {
-  return apiPost<Domain>("/api/domains", data);
+export async function createDomain(data: CreateDomainDto, workspaceId?: string): Promise<Domain> {
+  return apiPost<Domain>("/api/domains", data, workspaceId);
 }
 
 /**
  * Update a domain
  */
-export async function updateDomain(id: string, data: UpdateDomainDto): Promise<Domain> {
-  return apiPatch<Domain>(`/api/domains/${id}`, data);
+export async function updateDomain(
+  id: string,
+  data: UpdateDomainDto,
+  workspaceId?: string
+): Promise<Domain> {
+  return apiPatch<Domain>(`/api/domains/${id}`, data, workspaceId);
 }
 
 /**
  * Delete a domain
  */
-export async function deleteDomain(id: string): Promise<Record<string, never>> {
-  return apiDelete<Record<string, never>>(`/api/domains/${id}`);
+export async function deleteDomain(
+  id: string,
+  workspaceId?: string
+): Promise<Record<string, never>> {
+  return apiDelete<Record<string, never>>(`/api/domains/${id}`, workspaceId);
 }