diff --git a/src/ui/Logic/Download/SpellCheckDictionaryDownloadService.cs b/src/ui/Logic/Download/SpellCheckDictionaryDownloadService.cs
index 2fc8aba0652..4e41605a3ac 100644
--- a/src/ui/Logic/Download/SpellCheckDictionaryDownloadService.cs
+++ b/src/ui/Logic/Download/SpellCheckDictionaryDownloadService.cs
@@ -1,8 +1,10 @@
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Net.Http;
 using System.Threading;
 using System.Threading.Tasks;
+using Nikse.SubtitleEdit.UiLogic;
 
 namespace Nikse.SubtitleEdit.Logic.Download;
 
@@ -15,6 +17,26 @@ public class SpellCheckDictionaryDownloadService : ISpellCheckDictionaryDownload
 {
     private readonly HttpClient _httpClient;
 
+    internal const string VoikkoDllUrl =
+        "https://github.com/SubtitleEdit/support-files/releases/download/voikko-4.3-fi-2024-06/libvoikko-1.dll";
+    internal const string VoikkoDictionaryUrl =
+        "https://github.com/SubtitleEdit/support-files/releases/download/voikko-4.3-fi-2024-06/dict.zip";
+
+    internal const string VoikkoDllSha256 =
+        "bfffd537ff372b425a61940d4f5ac6c80e2a745dab33cc00ac50e8f50441d1b0";
+    internal const string VoikkoDictionarySha256 =
+        "98f26bb67e08288910fbf1aa92521f28bee79538ba86aefa267713333e7fa537";
+
+    private const string VoikkoReleasePathPrefix =
+        "/SubtitleEdit/support-files/releases/download/voikko-";
+
+    private static readonly IReadOnlyDictionary<string, string> VoikkoHashes =
+        new Dictionary<string, string>(StringComparer.Ordinal)
+        {
+            [VoikkoDllUrl] = VoikkoDllSha256,
+            [VoikkoDictionaryUrl] = VoikkoDictionarySha256,
+        };
+
     public SpellCheckDictionaryDownloadService(HttpClient httpClient)
     {
         _httpClient = httpClient;
@@ -22,6 +44,58 @@ public SpellCheckDictionaryDownloadService(HttpClient httpClient)
 
     public async Task DownloadDictionary(Stream stream, string url, IProgress<float>? progress, CancellationToken cancellationToken)
     {
+        var expected = GetExpectedVoikkoHash(url);
         await DownloadHelper.DownloadFileAsync(_httpClient, url, stream, progress, cancellationToken);
+
+        if (!string.IsNullOrEmpty(expected))
+        {
+            await VerifyVoikkoDownloadAsync(stream, expected, Path.GetFileName(new Uri(url).AbsolutePath), cancellationToken);
+        }
+    }
+
+    internal static string? GetExpectedVoikkoHash(string url)
+    {
+        if (VoikkoHashes.TryGetValue(url, out var expected))
+        {
+            return expected;
+        }
+
+        if (Uri.TryCreate(url, UriKind.Absolute, out var uri) &&
+            uri.AbsolutePath.StartsWith(VoikkoReleasePathPrefix, StringComparison.OrdinalIgnoreCase))
+        {
+            throw new InvalidOperationException($"No SHA-256 is registered for Voikko URL '{url}'.");
+        }
+
+        return null;
+    }
+
+    internal static async Task VerifyVoikkoDownloadAsync(
+        Stream stream,
+        string expected,
+        string fileName,
+        CancellationToken cancellationToken)
+    {
+        if (!stream.CanRead || !stream.CanSeek)
+        {
+            throw new InvalidOperationException("Voikko integrity verification requires a readable, seekable stream.");
+        }
+
+        string actual;
+        stream.Position = 0;
+        try
+        {
+            actual = await Sha256Util.ComputeSha256Async(stream, cancellationToken);
+        }
+        finally
+        {
+            stream.Position = 0;
+        }
+
+        if (!string.Equals(expected, actual, StringComparison.OrdinalIgnoreCase))
+        {
+            throw new IOException(
+                $"Voikko download failed integrity check for {fileName} " +
+                $"(expected SHA-256 {expected}, got {actual}).");
+        }
     }
-}
\ No newline at end of file
+}
diff --git a/tests/UI/Logic/Download/SpellCheckDictionaryDownloadServiceTests.cs b/tests/UI/Logic/Download/SpellCheckDictionaryDownloadServiceTests.cs
new file mode 100644
index 00000000000..9a17758fdd5
--- /dev/null
+++ b/tests/UI/Logic/Download/SpellCheckDictionaryDownloadServiceTests.cs
@@ -0,0 +1,118 @@
+using System.Net;
+using System.Text;
+using Nikse.SubtitleEdit.Logic.Download;
+
+namespace UITests.Logic.Download;
+
+public class SpellCheckDictionaryDownloadServiceTests
+{
+    [Theory]
+    [InlineData(
+        "https://github.com/SubtitleEdit/support-files/releases/download/voikko-4.3-fi-2024-06/libvoikko-1.dll",
+        "bfffd537ff372b425a61940d4f5ac6c80e2a745dab33cc00ac50e8f50441d1b0")]
+    [InlineData(
+        "https://github.com/SubtitleEdit/support-files/releases/download/voikko-4.3-fi-2024-06/dict.zip",
+        "98f26bb67e08288910fbf1aa92521f28bee79538ba86aefa267713333e7fa537")]
+    public void VoikkoUrl_MatchesPublishedReleaseDigest(string url, string expected)
+    {
+        Assert.Equal(expected, SpellCheckDictionaryDownloadService.GetExpectedVoikkoHash(url));
+    }
+
+    [Fact]
+    public void UnknownAssetOnPinnedVoikkoRelease_FailsClosed()
+    {
+        Assert.Throws<InvalidOperationException>(() =>
+            SpellCheckDictionaryDownloadService.GetExpectedVoikkoHash(
+                "https://github.com/SubtitleEdit/support-files/releases/download/voikko-4.3-fi-2024-06/future.zip"));
+    }
+
+    [Fact]
+    public void FutureVoikkoRelease_FailsClosedUntilHashesAreRegistered()
+    {
+        Assert.Throws<InvalidOperationException>(() =>
+            SpellCheckDictionaryDownloadService.GetExpectedVoikkoHash(
+                "https://github.com/SubtitleEdit/support-files/releases/download/voikko-5.0-fi-2026-09/dict.zip"));
+    }
+
+    [Fact]
+    public void PinnedVoikkoPathOnWrongOrigin_FailsClosed()
+    {
+        Assert.Throws<InvalidOperationException>(() =>
+            SpellCheckDictionaryDownloadService.GetExpectedVoikkoHash(
+                "https://example.invalid/SubtitleEdit/support-files/releases/download/voikko-4.3-fi-2024-06/dict.zip"));
+    }
+
+    [Fact]
+    public void GenericDictionaryUrl_IsNotForcedIntoVoikkoVerification()
+    {
+        Assert.Null(SpellCheckDictionaryDownloadService.GetExpectedVoikkoHash(
+            "https://example.invalid/dictionaries/dict.zip"));
+    }
+
+    [Fact]
+    public async Task DownloadDictionary_TamperedVoikkoPayload_IsRejectedAndRewound()
+    {
+        var handler = new StaticResponseHandler(Encoding.ASCII.GetBytes("tampered"));
+        using var httpClient = new HttpClient(handler);
+        var service = new SpellCheckDictionaryDownloadService(httpClient);
+        await using var stream = new MemoryStream();
+
+        await Assert.ThrowsAsync<IOException>(() =>
+            service.DownloadDictionary(
+                stream,
+                SpellCheckDictionaryDownloadService.VoikkoDictionaryUrl,
+                progress: null,
+                TestContext.Current.CancellationToken));
+
+        Assert.Equal(0, stream.Position);
+        Assert.Equal(new[] { HttpMethod.Head, HttpMethod.Get }, handler.RequestMethods);
+    }
+
+    [Fact]
+    public async Task VerifyVoikkoDownloadAsync_ValidPayload_RewindsStream()
+    {
+        await using var stream = new MemoryStream(Encoding.ASCII.GetBytes("abc"));
+        stream.Position = stream.Length;
+
+        await SpellCheckDictionaryDownloadService.VerifyVoikkoDownloadAsync(
+            stream,
+            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad",
+            "test.zip",
+            TestContext.Current.CancellationToken);
+
+        Assert.Equal(0, stream.Position);
+    }
+
+    [Fact]
+    public async Task GenericDictionaryDownload_RemainsUnchanged()
+    {
+        var payload = Encoding.ASCII.GetBytes("dictionary-data");
+        var handler = new StaticResponseHandler(payload);
+        using var httpClient = new HttpClient(handler);
+        var service = new SpellCheckDictionaryDownloadService(httpClient);
+        await using var stream = new MemoryStream();
+
+        await service.DownloadDictionary(
+            stream,
+            "https://example.invalid/dictionaries/pt_PT.dic",
+            progress: null,
+            TestContext.Current.CancellationToken);
+
+        Assert.Equal(payload.Length, stream.Length);
+        Assert.Equal(new[] { HttpMethod.Head, HttpMethod.Get }, handler.RequestMethods);
+    }
+
+    private sealed class StaticResponseHandler(byte[] payload) : HttpMessageHandler
+    {
+        public List<HttpMethod> RequestMethods { get; } = new();
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            RequestMethods.Add(request.Method);
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new ByteArrayContent(payload),
+            });
+        }
+    }
+}