diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 86058aac..64a7b49e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,6 +25,63 @@ jobs:
       matrix:
         platform: [linux-musl]
 
+  wasm-no-libc:
+    runs-on: ubuntu-latest
+    env:
+      CRATE_CC_NO_DEFAULTS: 1
+      CC_wasm32_unknown_unknown: /tmp/wasi-sdk-33.0-x86_64-linux/bin/clang --target=wasm32-unknown-unknown
+      AR_wasm32_unknown_unknown: /tmp/wasi-sdk-33.0-x86_64-linux/bin/llvm-ar
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          submodules: true
+      - run: rustup target add wasm32-unknown-unknown
+      - run: curl -fsSL https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-33/wasi-sdk-33.0-x86_64-linux.tar.gz | tar xz -C /tmp
+      - run: cargo build --target wasm32-unknown-unknown --no-default-features --features stock-zlib
+      - run: |
+          archive=$(find target/wasm32-unknown-unknown/debug/build -path '*/out/lib/libz.a' -print -quit)
+          test -n "$archive"
+          ! /tmp/wasi-sdk-33.0-x86_64-linux/bin/llvm-nm -u "$archive" | grep -Eq ' U (malloc|free)$'
+
+  wasm-wasi:
+    runs-on: ubuntu-latest
+    env:
+      CC_wasm32_wasip1: /tmp/wasi-sdk-33.0-x86_64-linux/bin/clang
+      AR_wasm32_wasip1: /tmp/wasi-sdk-33.0-x86_64-linux/bin/llvm-ar
+      CARGO_TARGET_WASM32_WASIP1_RUNNER: wasmtime
+      CC_wasm32_wasip1_threads: /tmp/wasi-sdk-33.0-x86_64-linux/bin/clang
+      AR_wasm32_wasip1_threads: /tmp/wasi-sdk-33.0-x86_64-linux/bin/llvm-ar
+      CC_wasm32_wasip2: /tmp/wasi-sdk-33.0-x86_64-linux/bin/clang
+      AR_wasm32_wasip2: /tmp/wasi-sdk-33.0-x86_64-linux/bin/llvm-ar
+      CARGO_TARGET_WASM32_WASIP2_RUNNER: wasmtime
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          submodules: true
+      - uses: bytecodealliance/actions/wasmtime/setup@9152e710e9f7182e4c29ad218e4f335a7b203613 # v1
+        with:
+          version: 46.0.1
+      - run: rustup target add wasm32-wasip1 wasm32-wasip1-threads wasm32-wasip2
+      - run: curl -fsSL https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-33/wasi-sdk-33.0-x86_64-linux.tar.gz | tar xz -C /tmp
+      - run: cargo test --target wasm32-wasip1 --test wasm
+      - run: cargo test --target wasm32-wasip2 --test wasm
+      # The threads ABI imports shared memory, which needs a custom host.
+      - run: cargo test --target wasm32-wasip1-threads --test wasm --no-run
+
+  wasm-emscripten:
+    runs-on: ubuntu-latest
+    env:
+      CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_RUNNER: node
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          submodules: true
+      - uses: emscripten-core/setup-emsdk@4528d102f7230f0e7b276855c01ea1159be0e984 # v16
+        with:
+          version: 6.0.4
+      - run: rustup target add wasm32-unknown-emscripten
+      - run: cargo test --target wasm32-unknown-emscripten --test wasm --features static
+
   windows:
     runs-on: ${{ matrix.runner || 'windows-latest' }}
     # Windows technically doesn't need this, but if we don't block windows on it
diff --git a/build.rs b/build.rs
index 60cabf0a..1a1f37a7 100644
--- a/build.rs
+++ b/build.rs
@@ -116,7 +116,13 @@ fn build_zlib(cfg: &mut cc::Build, target: &str) {
         .file("src/zlib/uncompr.c")
         .file("src/zlib/zutil.c");
 
-    if target.starts_with("wasm32") {
+    // Z_SOLO is zlib's "no C library at all" mode: it removes the default
+    // zalloc/zfree (so the one-shot compress()/uncompress() helpers return
+    // Z_STREAM_ERROR unless the caller wires up allocators by hand) and the
+    // gz* file API. Of the wasm32 targets only wasm32-unknown-unknown has no
+    // libc; the wasi targets (wasi-libc) and emscripten ship a complete C
+    // library, so they get the standard build.
+    if target == "wasm32-unknown-unknown" {
         cfg.define("Z_SOLO", None);
         // zlib 1.3.2 uses `NULL` directly in compress.c/uncompr.c, but the
         // Z_SOLO config path doesn't pull in headers that always define it.
diff --git a/tests/wasm.rs b/tests/wasm.rs
new file mode 100644
index 00000000..1a5dc4b9
--- /dev/null
+++ b/tests/wasm.rs
@@ -0,0 +1,35 @@
+#![cfg(any(target_os = "wasi", target_os = "emscripten"))]
+
+#[test]
+fn compress_roundtrip() {
+    let input = b"libz-sys on WebAssembly";
+    let mut compressed = vec![0; unsafe { libz_sys::compressBound(input.len() as _) } as usize];
+    let mut compressed_len = compressed.len() as _;
+
+    assert_eq!(
+        unsafe {
+            libz_sys::compress(
+                compressed.as_mut_ptr(),
+                &mut compressed_len,
+                input.as_ptr(),
+                input.len() as _,
+            )
+        },
+        libz_sys::Z_OK
+    );
+
+    let mut output = vec![0; input.len()];
+    let mut output_len = output.len() as _;
+    assert_eq!(
+        unsafe {
+            libz_sys::uncompress(
+                output.as_mut_ptr(),
+                &mut output_len,
+                compressed.as_ptr(),
+                compressed_len,
+            )
+        },
+        libz_sys::Z_OK
+    );
+    assert_eq!(&output[..output_len as usize], input);
+}