diff --git a/src/modules/courses/admin-course.controller.ts b/src/modules/courses/admin-course.controller.ts
index 0d895bb..7565c2e 100644
--- a/src/modules/courses/admin-course.controller.ts
+++ b/src/modules/courses/admin-course.controller.ts
@@ -10,6 +10,7 @@ import type {
   UpdateModuleBody,
   ModuleParams,
   ListEnrolledUsersQuery,
+  EnrollmentTrendsQuery,
 } from "./course.types.js";
 
 export class AdminCourseController {
@@ -225,6 +226,23 @@ export class AdminCourseController {
       },
     });
   }
+
+  /**
+   * GET /api/v1/admin/courses/:id/enrollment-trends
+   * Enrollment trends for a course over time (#391).
+   */
+  async enrollmentTrends(
+    request: FastifyRequest<{
+      Params: CourseIdParams;
+      Querystring: EnrollmentTrendsQuery;
+    }>,
+    reply: FastifyReply
+  ): Promise<void> {
+    const { id } = request.params;
+    const result = await courseService.getEnrollmentTrends(id, request.query);
+
+    reply.send({ success: true, data: result });
+  }
 }
 
 export const adminCourseController = new AdminCourseController();
diff --git a/src/modules/courses/admin-course.routes.ts b/src/modules/courses/admin-course.routes.ts
index 83dd31f..a009d85 100644
--- a/src/modules/courses/admin-course.routes.ts
+++ b/src/modules/courses/admin-course.routes.ts
@@ -10,6 +10,7 @@ import {
   updateModuleSchema,
   moduleParamsSchema,
   listEnrolledUsersQuerySchema,
+  enrollmentTrendsQuerySchema,
 } from "./course.types.js";
 
 /** Admin-only course management (#292). Every route requires an admin user. */
@@ -321,4 +322,34 @@ export async function adminCourseRoutes(app: FastifyInstance): Promise<void> {
     },
     (request, reply) => adminCourseController.listEnrolledUsers(request, reply)
   );
+
+  app.get<{
+    Params: { id: string };
+    Querystring: import("./course.types.js").EnrollmentTrendsQuery;
+  }>(
+    "/:id/enrollment-trends",
+    {
+      preHandler: [
+        validate({
+          params: courseIdParamsSchema,
+          querystring: enrollmentTrendsQuerySchema,
+        }),
+      ],
+      schema: {
+        description:
+          "Enrollment trends for a course over time with configurable range (7d/30d/90d) and granularity (daily/weekly/monthly) (admin only, cached 1 hour, #391)",
+        tags: ["admin", "courses"],
+        security: [{ bearerAuth: [] }],
+        params: { type: "object", required: ["id"], properties: { id: { type: "string", format: "uuid" } } },
+        querystring: {
+          type: "object",
+          properties: {
+            range: { type: "string", enum: ["7d", "30d", "90d"], default: "30d" },
+            granularity: { type: "string", enum: ["daily", "weekly", "monthly"], default: "daily" },
+          },
+        },
+      } as FastifySchema,
+    },
+    (request, reply) => adminCourseController.enrollmentTrends(request, reply)
+  );
 }
diff --git a/src/modules/courses/course.service.ts b/src/modules/courses/course.service.ts
index 8b30cf6..a92f5a7 100644
--- a/src/modules/courses/course.service.ts
+++ b/src/modules/courses/course.service.ts
@@ -57,6 +57,9 @@ import type {
   CourseAnalytics,
   EnrollmentTrendPoint,
   ModuleDifficulty,
+  EnrollmentTrendsQuery,
+  EnrollmentTrendsResult,
+  EnrollmentTrendDataPoint,
   CourseSyllabus,
   SyllabusModule,
 } from "./course.types.js";
@@ -2151,6 +2154,90 @@ export class CourseService {
       logger.warn({ err, courseId, reportId }, "Failed to notify admins of course report");
     }
   }
+
+  // ─── Enrollment Trends (#391) ───────────────────────────────────────────
+
+  /**
+   * Enrollment trends for a course over time (#391). Admin endpoint that
+   * returns a time series of enrollment counts at the chosen granularity
+   * (daily/weekly/monthly) within the chosen range (7d/30d/90d). Cached
+   * for 1 hour — matching getCourseAnalytics's TTL since the query shape
+   * is similar (aggregating across all enrollments for a course).
+   */
+  async getEnrollmentTrends(
+    courseId: string,
+    query: EnrollmentTrendsQuery,
+  ): Promise<EnrollmentTrendsResult> {
+    const course = await db.query.courses.findFirst({
+      where: eq(courses.id, courseId),
+    });
+    if (!course) {
+      throw new NotFoundError("Course");
+    }
+
+    const namespace = "courses";
+    const ck = cacheKey(
+      namespace,
+      "enrollment-trends",
+      courseId,
+      query.range,
+      query.granularity,
+    );
+    const cached = await cacheGet<EnrollmentTrendsResult>(namespace, ck);
+    if (cached) return cached;
+
+    const intervalMap: Record<string, string> = {
+      "7d": "7 days",
+      "30d": "30 days",
+      "90d": "90 days",
+    };
+    const truncMap: Record<string, string> = {
+      daily: "day",
+      weekly: "week",
+      monthly: "month",
+    };
+
+    const interval = intervalMap[query.range];
+    const trunc = truncMap[query.granularity];
+
+    const [trendRows] = await Promise.all([
+      db
+        .select({
+          date: sql<string>`date_trunc('${sql.raw(trunc)}', ${enrollments.enrolledAt})::date`,
+          count: count(),
+        })
+        .from(enrollments)
+        .where(
+          and(
+            eq(enrollments.courseId, courseId),
+            sql`${enrollments.enrolledAt} >= now() - interval '${sql.raw(interval)}'`,
+          ),
+        )
+        .groupBy(sql`date_trunc('${sql.raw(trunc)}', ${enrollments.enrolledAt})`)
+        .orderBy(sql`date_trunc('${sql.raw(trunc)}', ${enrollments.enrolledAt})`),
+      db
+        .select({ value: count() })
+        .from(enrollments)
+        .where(eq(enrollments.courseId, courseId)),
+    ]);
+
+    const trends: EnrollmentTrendDataPoint[] = trendRows.map((row) => ({
+      date: row.date,
+      count: row.count,
+    }));
+
+    const result: EnrollmentTrendsResult = {
+      courseId,
+      range: query.range,
+      granularity: query.granularity,
+      trends,
+      totalEnrollments: trendRows.reduce((sum, r) => sum + r.count, 0),
+      generatedAt: new Date(),
+    };
+
+    await cacheSet(ck, result, 3600);
+    return result;
+  }
 }
 
 export const courseService = new CourseService();
diff --git a/src/modules/courses/course.types.ts b/src/modules/courses/course.types.ts
index 96b2b28..e0fe23c 100644
--- a/src/modules/courses/course.types.ts
+++ b/src/modules/courses/course.types.ts
@@ -410,6 +410,26 @@ export interface CourseReportResult {
   createdAt: Date;
 }
 
+// ─── Enrollment Trends (#391) ───────────────────────────────────────────────
+
+export const enrollmentTrendsQuerySchema = z.object({
+  range: z.enum(["7d", "30d", "90d"]).default("30d"),
+  granularity: z.enum(["daily", "weekly", "monthly"]).default("daily"),
+});
+
+export type EnrollmentTrendsQuery = z.infer<typeof enrollmentTrendsQuerySchema>;
+
+export interface EnrollmentTrendDataPoint {
+  date: string;
+  count: number;
+}
+
+export interface EnrollmentTrendsResult {
+  courseId: string;
+  range: string;
+  granularity: string;
+  trends: EnrollmentTrendDataPoint[];
+  totalEnrollments: number;
 /** One module entry in the syllabus response. */
 export interface SyllabusModule {
   order: number;