diff --git a/SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs b/SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs
new file mode 100644
index 0000000..4e942c0
--- /dev/null
+++ b/SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs
@@ -0,0 +1,51 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using SVSim.Database;
+using SVSim.Database.Models;
+using SVSim.EmulatedEntrypoint.Models.Dtos.UserMyPage;
+
+namespace SVSim.EmulatedEntrypoint.Controllers;
+
+/// <summary>
+/// /user_mypage/* — viewer-scoped MyPage configuration writes. Separate from the
+/// <c>/mypage/*</c> family because the wire URL family is distinct.
+/// </summary>
+[Route("user_mypage")]
+public sealed class UserMyPageController : SVSimController
+{
+    private readonly SVSimDbContext _db;
+
+    public UserMyPageController(SVSimDbContext db) => _db = db;
+
+    [HttpPost("update")]
+    public async Task<ActionResult<UserMyPageUpdateResponse>> Update(
+        [FromBody] UserMyPageUpdateRequest request,
+        CancellationToken ct)
+    {
+        if (!TryGetViewerId(out var viewerId)) return Unauthorized();
+
+        var viewer = await _db.Viewers
+            .Include(v => v.MyPageBgRotation)
+            .FirstOrDefaultAsync(v => v.Id == viewerId, ct);
+        if (viewer is null) return NotFound();
+
+        viewer.MyPageBgSelectType = request.SelectType;
+        viewer.MyPageBgId = ParseIdOrZero(request.MyPageId);
+
+        viewer.MyPageBgRotation.Clear();
+        for (int slot = 0; slot < request.MyPageIdList.Count; slot++)
+        {
+            viewer.MyPageBgRotation.Add(new MyPageBgRotationEntry
+            {
+                Slot = slot,
+                BgId = ParseIdOrZero(request.MyPageIdList[slot]),
+            });
+        }
+
+        await _db.SaveChangesAsync(ct);
+        return new UserMyPageUpdateResponse();
+    }
+
+    private static int ParseIdOrZero(string s) =>
+        int.TryParse(s, out var n) ? n : 0;
+}
diff --git a/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateRequest.cs b/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateRequest.cs
new file mode 100644
index 0000000..6fbeec7
--- /dev/null
+++ b/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateRequest.cs
@@ -0,0 +1,28 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.UserMyPage;
+
+/// <summary>
+/// Body of <c>POST /user_mypage/update</c>. Client task: <c>MyPageSettingUpdateTask</c>
+/// (Shadowverse_Code_2026-05-23/Wizard/MyPageSettingUpdateTask.cs). Note that
+/// <c>select_type</c> is the only int on the wire — id fields are strings.
+/// </summary>
+[MessagePackObject]
+public sealed class UserMyPageUpdateRequest
+{
+    /// <summary>BGType enum: 0=Deck, 1=CustomBG, 2=RandomBG. Client sends as an int.</summary>
+    [JsonPropertyName("select_type")]
+    [Key("select_type")]
+    public int SelectType { get; set; }
+
+    /// <summary>Chosen BG id when SelectType=CustomBG; empty or "0" otherwise.</summary>
+    [JsonPropertyName("mypage_id")]
+    [Key("mypage_id")]
+    public string MyPageId { get; set; } = "0";
+
+    /// <summary>Saved rotation pool, in slot order; client sends the full list on every call.</summary>
+    [JsonPropertyName("mypage_id_list")]
+    [Key("mypage_id_list")]
+    public List<string> MyPageIdList { get; set; } = new();
+}
diff --git a/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateResponse.cs b/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateResponse.cs
new file mode 100644
index 0000000..481bc63
--- /dev/null
+++ b/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateResponse.cs
@@ -0,0 +1,12 @@
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.UserMyPage;
+
+/// <summary>
+/// Empty response payload. The client's <c>MyPageSettingUpdateTask.Parse()</c> is the default
+/// pass-through; server just acknowledges.
+/// </summary>
+[MessagePackObject(true)]
+public sealed class UserMyPageUpdateResponse
+{
+}
diff --git a/SVSim.UnitTests/Controllers/UserMyPageControllerTests.cs b/SVSim.UnitTests/Controllers/UserMyPageControllerTests.cs
new file mode 100644
index 0000000..fb20901
--- /dev/null
+++ b/SVSim.UnitTests/Controllers/UserMyPageControllerTests.cs
@@ -0,0 +1,149 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text;
+using System.Text.Json;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.Database;
+using SVSim.Database.Models;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Controllers;
+
+public class UserMyPageControllerTests
+{
+    private static StringContent JsonBody(string json) => new(json, Encoding.UTF8, "application/json");
+
+    private static async Task<Viewer> LoadViewerWithRotation(SVSimTestFactory factory, long viewerId)
+    {
+        using var scope = factory.Services.CreateScope();
+        var ctx = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        return await ctx.Viewers
+            .Include(v => v.MyPageBgRotation)
+            .AsNoTracking()
+            .FirstAsync(v => v.Id == viewerId);
+    }
+
+    [Test]
+    public async Task Update_persists_select_type_and_single_bg()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var body = JsonBody("""
+            {"select_type":1,"mypage_id":"1213410310","mypage_id_list":[]}
+            """);
+        var response = await client.PostAsync("/user_mypage/update", body);
+        var raw = await response.Content.ReadAsStringAsync();
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), raw);
+
+        var viewer = await LoadViewerWithRotation(factory, viewerId);
+        Assert.That(viewer.MyPageBgSelectType, Is.EqualTo(1));
+        Assert.That(viewer.MyPageBgId, Is.EqualTo(1213410310));
+        Assert.That(viewer.MyPageBgRotation, Is.Empty);
+    }
+
+    [Test]
+    public async Task Update_persists_rotation_pool_in_slot_order()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var body = JsonBody("""
+            {"select_type":2,"mypage_id":"0","mypage_id_list":["1211410310","1212410310","1213410310"]}
+            """);
+        var response = await client.PostAsync("/user_mypage/update", body);
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+
+        var viewer = await LoadViewerWithRotation(factory, viewerId);
+        var pool = viewer.MyPageBgRotation.OrderBy(r => r.Slot).Select(r => r.BgId).ToList();
+        Assert.That(pool, Is.EqualTo(new[] { 1211410310, 1212410310, 1213410310 }));
+    }
+
+    [Test]
+    public async Task Update_overwrites_previous_rotation_atomically()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+
+        // Seed a 5-entry rotation directly.
+        using (var seedScope = factory.Services.CreateScope())
+        {
+            var ctx = seedScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            var viewer = await ctx.Viewers.Include(v => v.MyPageBgRotation).FirstAsync(v => v.Id == viewerId);
+            for (int slot = 0; slot < 5; slot++)
+            {
+                viewer.MyPageBgRotation.Add(new MyPageBgRotationEntry { Slot = slot, BgId = 9000 + slot });
+            }
+            await ctx.SaveChangesAsync();
+        }
+
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+        var body = JsonBody("""
+            {"select_type":2,"mypage_id":"0","mypage_id_list":["1001","1002","1003"]}
+            """);
+        var response = await client.PostAsync("/user_mypage/update", body);
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+
+        var viewer2 = await LoadViewerWithRotation(factory, viewerId);
+        var pool = viewer2.MyPageBgRotation.OrderBy(r => r.Slot).Select(r => r.BgId).ToList();
+        Assert.That(pool, Is.EqualTo(new[] { 1001, 1002, 1003 }),
+            "old slots 3-4 should have been deleted, not orphaned");
+    }
+
+    [Test]
+    public async Task Update_with_empty_mypage_id_falls_back_to_zero()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var body = JsonBody("""
+            {"select_type":0,"mypage_id":"","mypage_id_list":[]}
+            """);
+        var response = await client.PostAsync("/user_mypage/update", body);
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+
+        var viewer = await LoadViewerWithRotation(factory, viewerId);
+        Assert.That(viewer.MyPageBgSelectType, Is.EqualTo(0));
+        Assert.That(viewer.MyPageBgId, Is.EqualTo(0));
+    }
+
+    [Test]
+    public async Task Update_with_unparseable_mypage_id_falls_back_to_zero()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var body = JsonBody("""
+            {"select_type":0,"mypage_id":"garbage","mypage_id_list":[]}
+            """);
+        var response = await client.PostAsync("/user_mypage/update", body);
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+
+        var viewer = await LoadViewerWithRotation(factory, viewerId);
+        Assert.That(viewer.MyPageBgId, Is.EqualTo(0));
+    }
+
+    [Test]
+    public async Task Update_returns_envelope_with_empty_data_payload()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var body = JsonBody("""
+            {"select_type":0,"mypage_id":"0","mypage_id_list":[]}
+            """);
+        var response = await client.PostAsync("/user_mypage/update", body);
+        var raw = await response.Content.ReadAsStringAsync();
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), raw);
+
+        // Test path bypasses the translation middleware (gated on UnityPlayer UA), so the
+        // controller's literal return value is what comes back. An empty class serializes to "{}".
+        Assert.That(raw, Is.EqualTo("{}"));
+    }
+}