diff --git a/lib/std/Io/Threaded/test.zig b/lib/std/Io/Threaded/test.zig
index 0e5d90e6d4bdbf8363d8f48ce0abbf03cc3f662d..9ab9c6dcd98b51414611c7e492663d6e01aa795f 100644
--- a/lib/std/Io/Threaded/test.zig
+++ b/lib/std/Io/Threaded/test.zig
@@ -206,6 +206,11 @@ test "cancel blocked read from pipe" {
 }
 
 test "memory mapping fallback" {
+    if (builtin.os.tag == .wasi and builtin.link_libc) {
+        // https://github.com/ziglang/zig/issues/20747 (open fd does not have write permission)
+        return error.SkipZigTest;
+    }
+
     var threaded: std.Io.Threaded = .init(std.testing.allocator, .{
         .argv0 = .empty,
         .environ = .empty,
diff --git a/lib/std/Io/test.zig b/lib/std/Io/test.zig
index e1c5bfea83934ae74e4754a28220ff95f12f873c..00a2d872418167d53c5d09d3ce2d8072d21026fa 100644
--- a/lib/std/Io/test.zig
+++ b/lib/std/Io/test.zig
@@ -595,6 +595,10 @@ test "randomSecure" {
 
 test "memory mapping" {
     if (builtin.cpu.arch == .hexagon) return error.SkipZigTest; // mmap returned EINVAL
+    if (builtin.os.tag == .wasi and builtin.link_libc) {
+        // https://github.com/ziglang/zig/issues/20747 (open fd does not have write permission)
+        return error.SkipZigTest;
+    }
 
     const io = testing.io;