diff --git a/eng/pipelines/libraries/fuzzing/deploy-to-onefuzz.yml b/eng/pipelines/libraries/fuzzing/deploy-to-onefuzz.yml
index 85c92f292323f9..7b3319c28f12cd 100644
--- a/eng/pipelines/libraries/fuzzing/deploy-to-onefuzz.yml
+++ b/eng/pipelines/libraries/fuzzing/deploy-to-onefuzz.yml
@@ -207,4 +207,12 @@ extends:
             onefuzzDropDirectory: $(fuzzerProject)/deployment/ZipArchiveFuzzer
             SYSTEM_ACCESSTOKEN: $(System.AccessToken)
           displayName: Send ZipArchiveFuzzer to OneFuzz
+
+        - task: onefuzz-task@0
+          inputs:
+            onefuzzOSes: 'Windows'
+          env:
+            onefuzzDropDirectory: $(fuzzerProject)/deployment/ZipEncryptionStreamFuzzer
+            SYSTEM_ACCESSTOKEN: $(System.AccessToken)
+          displayName: Send ZipEncryptionStreamFuzzer to OneFuzz
 # ONEFUZZ_TASK_WORKAROUND_END
diff --git a/src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/ZipEncryptionStreamFuzzer.cs b/src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/ZipEncryptionStreamFuzzer.cs
new file mode 100644
index 00000000000000..b3414b743473e9
--- /dev/null
+++ b/src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/ZipEncryptionStreamFuzzer.cs
@@ -0,0 +1,197 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Buffers;
+using System.IO.Compression;
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+using System.Threading.Tasks;
+
+namespace DotnetFuzzing.Fuzzers;
+
+// Exercises every ZIP entry-encryption method through a single round-trip: legacy ZipCrypto
+// (ZipCryptoStream) and WinZip AES-128/192/256 (WinZipAesStream). The two ciphers share the same
+// ZipArchiveEntry entry points and stream shape, so one fuzzer covers both encryption paths.
+[UnsupportedOSPlatform("browser")]
+internal sealed class ZipEncryptionStreamFuzzer : IFuzzer
+{
+    public string[] TargetAssemblies { get; } = ["System.IO.Compression"];
+    public string[] TargetCoreLibPrefixes => [];
+
+    // The entry name is not part of the encryption-stream surface this fuzzer targets: it only feeds
+    // the generic ZIP name-encoding logic (ASCII-vs-UTF8 detection and the UTF-8 general-purpose-bit
+    // flag), which is independent of the AES/ZipCrypto encryption path. Fuzzing it here would add
+    // coverage outside the target, so it is hardcoded; entry-name encoding belongs in a ZIP fuzzer.
+    private const string EntryName = "entry";
+
+    // Used only when the fuzzed password slice is empty; a non-empty password is required to encrypt.
+    private const string FallbackPassword = "fuzz-password";
+
+    private static readonly ZipEncryptionMethod[] s_methods =
+    [
+        ZipEncryptionMethod.ZipCrypto,
+        ZipEncryptionMethod.Aes128,
+        ZipEncryptionMethod.Aes192,
+        ZipEncryptionMethod.Aes256,
+    ];
+
+    public void FuzzTarget(ReadOnlySpan<byte> bytes)
+    {
+        // The first byte is consumed below as the encryption-method selector, so an empty
+        // input has no byte to interpret and there is nothing to fuzz.
+        if (bytes.IsEmpty)
+        {
+            return;
+        }
+
+        // Use the first byte to select the encryption method so all variants get exercised.
+        ZipEncryptionMethod method = s_methods[bytes[0] % s_methods.Length];
+        ReadOnlySpan<byte> payload = bytes.Slice(1);
+
+        // Split the rest of the input into a password and the entry content: everything up to the
+        // first newline is the password, the remainder is the content. Deriving the password from the
+        // input drives the key derivation and the UTF-8 password-encoding path instead of always using
+        // a fixed key. The content — the remainder — is what drives the cipher chunking (and, for AES,
+        // the trailing HMAC), so it stays the bulk of the input.
+        int newlineIndex = payload.IndexOf((byte)'\n');
+        ReadOnlySpan<byte> passwordBytes = newlineIndex < 0 ? ReadOnlySpan<byte>.Empty : payload.Slice(0, newlineIndex);
+        ReadOnlySpan<byte> contentBytes = newlineIndex < 0 ? payload : payload.Slice(newlineIndex + 1);
+
+        // An empty entry is stored unencrypted (no ciphertext is produced and IsEncrypted is false on
+        // read-back), so it does not exercise the encryption stream this fuzzer targets. Skip it.
+        if (contentBytes.IsEmpty)
+        {
+            return;
+        }
+
+        // Interpret the password bytes as UTF-16 so arbitrary char values (including lone surrogates)
+        // exercise the UTF-8 encoding of the password during key derivation.
+        string password = new string(MemoryMarshal.Cast<byte, char>(passwordBytes));
+
+        // A non-empty password is required to create an encrypted entry (CreateEntry throws otherwise).
+        // The fuzzed slice may yield an empty string (no newline, or a single byte that casts to zero
+        // chars), so fall back to a fixed non-empty password to keep exercising the key-derivation path.
+        if (password.Length == 0)
+        {
+            password = FallbackPassword;
+        }
+
+        // Poison the page immediately after the content so any over-read while encrypting or
+        // decrypting faults immediately instead of silently reading adjacent memory.
+        using PooledBoundedMemory<byte> buffer = PooledBoundedMemory<byte>.Rent(contentBytes, PoisonPagePlacement.After);
+        RoundTrip(buffer.Memory, password, method, async: false).GetAwaiter().GetResult();
+        RoundTrip(buffer.Memory, password, method, async: true).GetAwaiter().GetResult();
+    }
+
+    private static async Task RoundTrip(ReadOnlyMemory<byte> content, string password, ZipEncryptionMethod method, bool async)
+    {
+        using var archiveStream = new MemoryStream();
+
+        // Encrypt the fuzz input, exercising the encryption stream (and HMAC path for AES).
+        ZipArchive writeArchive = async
+            ? await ZipArchive.CreateAsync(archiveStream, ZipArchiveMode.Create, leaveOpen: true, entryNameEncoding: null)
+            : new ZipArchive(archiveStream, ZipArchiveMode.Create, leaveOpen: true, entryNameEncoding: null);
+
+        ZipArchiveEntry writeEntry = writeArchive.CreateEntry(EntryName, password.AsSpan(), method);
+
+        Stream entryWriteStream = async ? await writeEntry.OpenAsync() : writeEntry.Open();
+        if (async)
+        {
+            await entryWriteStream.WriteAsync(content);
+            await entryWriteStream.DisposeAsync();
+            await writeArchive.DisposeAsync();
+        }
+        else
+        {
+            entryWriteStream.Write(content.Span);
+            entryWriteStream.Dispose();
+            writeArchive.Dispose();
+        }
+
+        archiveStream.Position = 0;
+
+        // Decrypt with the correct password and verify the round-trip is lossless.
+        ZipArchive readArchive = async
+            ? await ZipArchive.CreateAsync(archiveStream, ZipArchiveMode.Read, leaveOpen: true, entryNameEncoding: null)
+            : new ZipArchive(archiveStream, ZipArchiveMode.Read, leaveOpen: true, entryNameEncoding: null);
+
+        ZipArchiveEntry readEntry = readArchive.GetEntry(EntryName)!;
+        Assert.True(readEntry.IsEncrypted);
+        Assert.Equal(method, readEntry.EncryptionMethod);
+
+        using (var decrypted = new MemoryStream())
+        {
+            Stream entryReadStream = async ? await readEntry.OpenAsync(password.AsSpan()) : readEntry.Open(password.AsSpan());
+            if (async)
+            {
+                await entryReadStream.CopyToAsync(decrypted);
+                await entryReadStream.DisposeAsync();
+            }
+            else
+            {
+                entryReadStream.CopyTo(decrypted);
+                entryReadStream.Dispose();
+            }
+
+            Assert.SequenceEqual(content.Span, decrypted.GetBuffer().AsSpan(0, (int)decrypted.Length));
+        }
+
+        // AES is authenticated: the password verifier and HMAC make accepting a wrong key
+        // cryptographically infeasible, so a wrong password must fail with InvalidDataException.
+        // ZipCrypto only has a weak 1-byte verifier over a stream cipher, so a wrong key is rejected
+        // with roughly 255/256 probability and may occasionally decrypt to garbage without throwing;
+        // both outcomes are acceptable and cannot be distinguished reliably on fuzzed input. Any
+        // other exception type would indicate a real bug and is intentionally left to propagate.
+        // Append a byte so the wrong password is guaranteed to differ from the fuzzed one above.
+        bool authenticated = method != ZipEncryptionMethod.ZipCrypto;
+        string wrongPassword = password + "!";
+        bool wrongPasswordRejected = false;
+        try
+        {
+            Stream wrongStream = async
+                ? await readEntry.OpenAsync(wrongPassword.AsSpan())
+                : readEntry.Open(wrongPassword.AsSpan());
+            try
+            {
+                if (async)
+                {
+                    await wrongStream.CopyToAsync(Stream.Null);
+                }
+                else
+                {
+                    wrongStream.CopyTo(Stream.Null);
+                }
+            }
+            finally
+            {
+                if (async)
+                {
+                    await wrongStream.DisposeAsync();
+                }
+                else
+                {
+                    wrongStream.Dispose();
+                }
+            }
+        }
+        catch (InvalidDataException)
+        {
+            // Expected: the AES password verifier / HMAC (or the ZipCrypto check byte) rejects the wrong key.
+            wrongPasswordRejected = true;
+        }
+
+        if (authenticated)
+        {
+            Assert.True(wrongPasswordRejected);
+        }
+
+        if (async)
+        {
+            await readArchive.DisposeAsync();
+        }
+        else
+        {
+            readArchive.Dispose();
+        }
+    }
+}