diff --git a/backend/src/CCE.Api.Common/Localization/Resources.yaml b/backend/src/CCE.Api.Common/Localization/Resources.yaml
index 3bac7e4..31e84ff 100644
--- a/backend/src/CCE.Api.Common/Localization/Resources.yaml
+++ b/backend/src/CCE.Api.Common/Localization/Resources.yaml
@@ -335,3 +335,11 @@ SCENARIO_NOT_FOUND:
 TECHNOLOGY_NOT_FOUND:
   ar: "التقنية غير موجودة"
   en: "Technology not found"
+
+INTEREST_NOT_FOUND:
+  ar: "الاهتمام غير موجود"
+  en: "Interest not found"
+
+INTEREST_UPSERTED:
+  ar: "تم تحديث الاهتمامات بنجاح"
+  en: "Interests updated successfully"
diff --git a/backend/src/CCE.Api.External/Endpoints/UserInterestEndpoints.cs b/backend/src/CCE.Api.External/Endpoints/UserInterestEndpoints.cs
new file mode 100644
index 0000000..817ecab
--- /dev/null
+++ b/backend/src/CCE.Api.External/Endpoints/UserInterestEndpoints.cs
@@ -0,0 +1,36 @@
+using CCE.Api.Common.Extensions;
+using CCE.Application.Common.Interfaces;
+using CCE.Application.Identity.Public.Commands.UserInterest;
+using MediatR;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Routing;
+
+namespace CCE.Api.External.Endpoints;
+
+public static class UserInterestEndpoints
+{
+    public static IEndpointRouteBuilder MapUserInterestEndpoints(this IEndpointRouteBuilder app)
+    {
+        var me = app.MapGroup("/api/me").WithTags("User Interests").RequireAuthorization();
+
+        me.MapPatch("/interests", async (
+            UpsertUserInterestRequest body,
+            ICurrentUserAccessor currentUser,
+            IMediator mediator,
+            CancellationToken ct) =>
+        {
+            var userId = currentUser.GetUserId() ?? System.Guid.Empty;
+            if (userId == System.Guid.Empty) return Results.Unauthorized();
+
+            var result = await mediator.Send(
+                new UpsertUserInterestCommand(userId, body.Interests), ct).ConfigureAwait(false);
+            return result.ToHttpResult();
+        })
+        .WithName("UpsertUserInterest");
+
+        return app;
+    }
+}
+
+public sealed record UpsertUserInterestRequest(IReadOnlyList<string> Interests);
diff --git a/backend/src/CCE.Api.External/Program.cs b/backend/src/CCE.Api.External/Program.cs
index f2439ee..3ebb9b7 100644
--- a/backend/src/CCE.Api.External/Program.cs
+++ b/backend/src/CCE.Api.External/Program.cs
@@ -103,6 +103,7 @@
 app.MapAssistantEndpoints();
 app.MapKapsarcEndpoints();
 app.MapSurveysEndpoints();
+app.MapUserInterestEndpoints();
 
 app.MapGet("/health", async (IMediator mediator) =>
 {
diff --git a/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestCommand.cs b/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestCommand.cs
new file mode 100644
index 0000000..10d74bb
--- /dev/null
+++ b/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestCommand.cs
@@ -0,0 +1,8 @@
+using CCE.Application.Common;
+using MediatR;
+
+namespace CCE.Application.Identity.Public.Commands.UserInterest;
+
+public sealed record UpsertUserInterestCommand(
+    System.Guid UserId,
+    IReadOnlyList<string> Interests) : IRequest<Response<UpsertUserInterestResult>>;
diff --git a/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestCommandHandler.cs b/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestCommandHandler.cs
new file mode 100644
index 0000000..6acd734
--- /dev/null
+++ b/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestCommandHandler.cs
@@ -0,0 +1,67 @@
+using System.Linq;
+using CCE.Application.Common;
+using CCE.Application.Common.Interfaces;
+using CCE.Application.Messages;
+using MediatR;
+
+namespace CCE.Application.Identity.Public.Commands.UserInterest;
+
+public sealed class UpsertUserInterestCommandHandler
+    : IRequestHandler<UpsertUserInterestCommand, Response<UpsertUserInterestResult>>
+{
+    private readonly IUserProfileRepository _service;
+    private readonly ICceDbContext _db;
+    private readonly MessageFactory _msg;
+
+    public UpsertUserInterestCommandHandler(
+        IUserProfileRepository service,
+        ICceDbContext db,
+        MessageFactory msg)
+    {
+        _service = service;
+        _db = db;
+        _msg = msg;
+    }
+
+    public async Task<Response<UpsertUserInterestResult>> Handle(
+        UpsertUserInterestCommand request,
+        CancellationToken cancellationToken)
+    {
+        var user = await _service.FindAsync(request.UserId, cancellationToken).ConfigureAwait(false);
+        if (user is null)
+            return _msg.UserNotFound<UpsertUserInterestResult>();
+
+        var oldInterests = user.Interests.ToList();
+        var rawList = request.Interests ?? System.Array.Empty<string>();
+
+        var normalizedNew = rawList
+            .Select(static s => s?.Trim() ?? string.Empty)
+            .Where(static s => s.Length > 0)
+            .Distinct(StringComparer.OrdinalIgnoreCase)
+            .ToList();
+
+        var oldSet = new HashSet<string>(oldInterests, StringComparer.OrdinalIgnoreCase);
+        var newSet = new HashSet<string>(normalizedNew, StringComparer.OrdinalIgnoreCase);
+
+        if (oldSet.SetEquals(newSet))
+        {
+            return _msg.InterestUpserted(new UpsertUserInterestResult(
+                user.Interests,
+                System.Array.Empty<string>(),
+                System.Array.Empty<string>()));
+        }
+
+        user.UpdateInterests(normalizedNew);
+
+        var added = normalizedNew.Except(oldInterests, StringComparer.OrdinalIgnoreCase).ToList();
+        var removed = oldInterests.Except(normalizedNew, StringComparer.OrdinalIgnoreCase).ToList();
+
+        _service.Update(user);
+        await _db.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
+
+        return _msg.InterestUpserted(new UpsertUserInterestResult(
+            user.Interests,
+            added,
+            removed));
+    }
+}
diff --git a/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestResult.cs b/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestResult.cs
new file mode 100644
index 0000000..8e55310
--- /dev/null
+++ b/backend/src/CCE.Application/Identity/Public/Commands/UserInterest/UpsertUserInterestResult.cs
@@ -0,0 +1,6 @@
+namespace CCE.Application.Identity.Public.Commands.UserInterest;
+
+public sealed record UpsertUserInterestResult(
+    IReadOnlyList<string> Interests,
+    IReadOnlyList<string> Added,
+    IReadOnlyList<string> Removed);
diff --git a/backend/src/CCE.Application/Messages/MessageFactory.cs b/backend/src/CCE.Application/Messages/MessageFactory.cs
index 6ba4868..b1742ce 100644
--- a/backend/src/CCE.Application/Messages/MessageFactory.cs
+++ b/backend/src/CCE.Application/Messages/MessageFactory.cs
@@ -68,6 +68,7 @@ public FieldError Field(string fieldName, string domainKey)
     // ─── Convenience shortcuts (Identity domain) ───
 
     public Response<T> UserNotFound<T>()      => NotFound<T>("USER_NOT_FOUND");
+    public Response<T> InterestUpserted<T>(T data) => Ok(data, "INTEREST_UPSERTED");
     public Response<T> EmailExists<T>()       => Conflict<T>("EMAIL_EXISTS");
     public Response<T> InvalidCredentials<T>() => Unauthorized<T>("INVALID_CREDENTIALS");
     public Response<T> NotAuthenticated<T>()  => Unauthorized<T>("NOT_AUTHENTICATED");
diff --git a/backend/src/CCE.Application/Messages/SystemCode.cs b/backend/src/CCE.Application/Messages/SystemCode.cs
index beb7aab..da99ec1 100644
--- a/backend/src/CCE.Application/Messages/SystemCode.cs
+++ b/backend/src/CCE.Application/Messages/SystemCode.cs
@@ -21,6 +21,7 @@ public static class SystemCode
     public const string ERR002 = "ERR002"; // Resource download failure (appendix)
     public const string ERR003 = "ERR003"; // Resource share failure (appendix)
 
+    public const string ERR018 = "ERR018"; // Interest not found
     public const string ERR019 = "ERR019"; // Email already exists / Account creation failure (appendix)
     public const string ERR020 = "ERR020"; // Invalid credentials (appendix)
     public const string ERR021 = "ERR021"; // Login system error (appendix)
@@ -115,6 +116,7 @@ public static class SystemCode
     public const string CON007 = "CON007"; // Admin notified of expert request (appendix)
     public const string CON008 = "CON008"; // Service evaluation submitted (appendix)
     public const string CON009 = "CON009"; // Personalized suggestions submitted (appendix)
+    public const string CON018 = "CON018"; // Interest upserted
     public const string CON010 = "CON010"; // Topic follow success (appendix)
     public const string CON011 = "CON011"; // Post created (appendix)
     public const string CON012 = "CON012"; // Post follow success (appendix)
diff --git a/backend/src/CCE.Application/Messages/SystemCodeMap.cs b/backend/src/CCE.Application/Messages/SystemCodeMap.cs
index f53ae1a..c5f4d60 100644
--- a/backend/src/CCE.Application/Messages/SystemCodeMap.cs
+++ b/backend/src/CCE.Application/Messages/SystemCodeMap.cs
@@ -14,6 +14,7 @@ public static class SystemCodeMap
         ["INVALID_CREDENTIALS"] = SystemCode.ERR020,
         ["PASSWORD_RECOVERY_FAILED"] = SystemCode.ERR023,
         ["LOGOUT_FAILED"] = SystemCode.ERR024,
+        ["INTEREST_NOT_FOUND"] = SystemCode.ERR018,
 
         // ─── Backend-only Identity Errors (moved to free appendix numbers) ───
         ["EXPERT_REQUEST_NOT_FOUND"] = SystemCode.ERR400,
@@ -90,6 +91,7 @@ public static class SystemCodeMap
         ["PASSWORD_RESET"] = SystemCode.CON014,
         ["LOGOUT_SUCCESS"] = SystemCode.CON015,
         ["REGISTER_SUCCESS"] = SystemCode.CON017,
+        ["INTEREST_UPSERTED"] = SystemCode.CON018,
 
         // ─── Backend-only Identity Success (appendix numbers already taken) ───
         ["EXPERT_REQUEST_APPROVED"] = SystemCode.CON050,