diff --git a/crates/cli-support/src/lib.rs b/crates/cli-support/src/lib.rs
index abe5ec226ee..ea165386ddc 100755
--- a/crates/cli-support/src/lib.rs
+++ b/crates/cli-support/src/lib.rs
@@ -338,8 +338,15 @@ impl Bindgen {
                 .context("failed getting Wasm module")?,
         };
 
-        self.threads
-            .run(&mut module)
+        let mut start = module.start.take().map(|start| {
+            let mut builder = walrus::FunctionBuilder::new(&mut module.types, &[], &[]);
+            builder.func_body().call(start);
+            builder
+        });
+
+        let thread_count = self
+            .threads
+            .run(&mut module, &mut start)
             .with_context(|| "failed to prepare module for threading")?;
 
         // If requested, turn all mangled symbols into prettier unmangled
@@ -380,9 +387,11 @@ impl Bindgen {
         // interface types.
         wit::process(
             &mut module,
+            &mut start,
             programs,
             self.externref,
             self.wasm_interface_types,
+            thread_count,
             self.emit_start,
         )?;
 
@@ -435,6 +444,8 @@ impl Bindgen {
                 .context("failed to transform return pointers into multi-value Wasm")?;
         }
 
+        module.start = start.map(|builder| builder.finish(Vec::new(), &mut module.funcs));
+
         // We've done a whole bunch of transformations to the wasm module, many
         // of which leave "garbage" lying around, so let's prune out all our
         // unnecessary things here.
diff --git a/crates/cli-support/src/wit/mod.rs b/crates/cli-support/src/wit/mod.rs
index d098eab93bc..82d34d38e31 100644
--- a/crates/cli-support/src/wit/mod.rs
+++ b/crates/cli-support/src/wit/mod.rs
@@ -8,6 +8,7 @@ use std::str;
 use walrus::MemoryId;
 use walrus::{ExportId, FunctionId, ImportId, Module};
 use wasm_bindgen_shared::struct_function_export_name;
+use wasm_bindgen_threads_xform::ThreadCount;
 
 mod incoming;
 mod nonstandard;
@@ -20,6 +21,7 @@ pub use self::standard::*;
 struct Context<'a> {
     start_found: bool,
     module: &'a mut Module,
+    start: &'a mut Option<walrus::FunctionBuilder>,
     adapters: NonstandardWitSection,
     aux: WasmBindgenAux,
     /// All of the wasm module's exported functions.
@@ -34,6 +36,7 @@ struct Context<'a> {
     descriptors: HashMap<String, Descriptor>,
     externref_enabled: bool,
     wasm_interface_types: bool,
+    thread_count: Option<ThreadCount>,
     support_start: bool,
 }
 
@@ -47,9 +50,11 @@ struct InstructionBuilder<'a, 'b> {
 
 pub fn process(
     module: &mut Module,
+    start: &mut Option<walrus::FunctionBuilder>,
     programs: Vec<decode::Program>,
     externref_enabled: bool,
     wasm_interface_types: bool,
+    thread_count: Option<ThreadCount>,
     support_start: bool,
 ) -> Result<(NonstandardWitSectionId, WasmBindgenAuxId), Error> {
     let mut cx = Context {
@@ -63,9 +68,11 @@ pub fn process(
         unique_crate_identifier: "",
         memory: wasm_bindgen_wasm_conventions::get_memory(module).ok(),
         module,
+        start,
         start_found: false,
         externref_enabled,
         wasm_interface_types,
+        thread_count,
         support_start,
     };
     cx.init()?;
@@ -312,14 +319,13 @@ impl<'a> Context<'a> {
             self.module
                 .add_import_func(PLACEHOLDER_MODULE, "__wbindgen_init_externref_table", ty);
 
-        self.module.start = Some(match self.module.start {
-            Some(prev_start) => {
-                let mut builder = walrus::FunctionBuilder::new(&mut self.module.types, &[], &[]);
-                builder.func_body().call(import).call(prev_start);
-                builder.finish(Vec::new(), &mut self.module.funcs)
-            }
-            None => import,
-        });
+        let module = &mut self.module;
+        let builder = self
+            .start
+            .get_or_insert_with(|| walrus::FunctionBuilder::new(&mut module.types, &[], &[]));
+
+        builder.func_body().call_at(0, import);
+
         self.bind_intrinsic(import_id, Intrinsic::InitExternrefTable)?;
 
         Ok(())
@@ -481,22 +487,21 @@ impl<'a> Context<'a> {
             return Ok(());
         }
 
-        let prev_start = match self.module.start {
-            Some(f) => f,
-            None => {
-                self.module.start = Some(id);
-                return Ok(());
-            }
-        };
+        let module = &mut self.module;
+        let builder = self
+            .start
+            .get_or_insert_with(|| walrus::FunctionBuilder::new(&mut module.types, &[], &[]));
 
-        // Note that we call the previous start function, if any, first. This is
+        // Note that we leave the previous start function, if any, first. This is
         // because the start function currently only shows up when it's injected
         // through thread/externref transforms. These injected start functions
         // need to happen before user code, so we always schedule them first.
-        let mut builder = walrus::FunctionBuilder::new(&mut self.module.types, &[], &[]);
-        builder.func_body().call(prev_start).call(id);
-        let new_start = builder.finish(Vec::new(), &mut self.module.funcs);
-        self.module.start = Some(new_start);
+        if let Some(thread_count) = self.thread_count {
+            thread_count.wrap_start(builder, id)
+        } else {
+            builder.func_body().call(id);
+        }
+
         Ok(())
     }
 
diff --git a/crates/threads-xform/src/lib.rs b/crates/threads-xform/src/lib.rs
index 5f8361c21b0..169f0eaebc5 100644
--- a/crates/threads-xform/src/lib.rs
+++ b/crates/threads-xform/src/lib.rs
@@ -23,6 +23,9 @@ pub struct Config {
     enabled: bool,
 }
 
+#[derive(Clone, Copy)]
+pub struct ThreadCount(walrus::LocalId);
+
 impl Config {
     /// Create a new configuration with default settings.
     pub fn new() -> Config {
@@ -103,9 +106,13 @@ impl Config {
     /// * Some stack space is prepared for each thread after the first one.
     ///
     /// More and/or less may happen here over time, stay tuned!
-    pub fn run(&self, module: &mut Module) -> Result<(), Error> {
+    pub fn run(
+        &self,
+        module: &mut Module,
+        start: &mut Option<walrus::FunctionBuilder>,
+    ) -> Result<Option<ThreadCount>, Error> {
         if !self.is_enabled(module) {
-            return Ok(());
+            return Ok(None);
         }
 
         let memory = wasm_conventions::get_memory(module)?;
@@ -157,7 +164,7 @@ impl Config {
 
         let _ = module.exports.add("__stack_alloc", stack.alloc);
 
-        inject_start(module, &tls, &stack, thread_counter_addr, memory)?;
+        let thread_count = inject_start(module, start, &tls, &stack, thread_counter_addr, memory)?;
 
         // we expose a `__wbindgen_thread_destroy()` helper function that deallocates stack space.
         //
@@ -177,7 +184,21 @@ impl Config {
         //   call while the leader is destroying its stack! You should make sure that this cannot happen.
         inject_destroy(module, &tls, &stack, memory)?;
 
-        Ok(())
+        Ok(Some(thread_count))
+    }
+}
+
+impl ThreadCount {
+    pub fn wrap_start(self, builder: &mut walrus::FunctionBuilder, start: FunctionId) {
+        // We only want to call the start function if we are in the first thread.
+        // The thread counter should be 0 for the first thread.
+        builder.func_body().local_get(self.0).if_else(
+            None,
+            |_| {},
+            |body| {
+                body.call(start);
+            },
+        );
     }
 }
 
@@ -296,26 +317,22 @@ struct Stack {
 
 fn inject_start(
     module: &mut Module,
+    start: &mut Option<walrus::FunctionBuilder>,
     tls: &Tls,
     stack: &Stack,
     thread_counter_addr: i32,
     memory: MemoryId,
-) -> Result<(), Error> {
+) -> Result<ThreadCount, Error> {
     use walrus::ir::*;
 
     assert!(stack.size % PAGE_SIZE == 0);
-    let mut builder = walrus::FunctionBuilder::new(&mut module.types, &[], &[]);
+    let builder =
+        start.get_or_insert_with(|| walrus::FunctionBuilder::new(&mut module.types, &[], &[]));
     let local = module.locals.add(ValType::I32);
+    let thread_count = module.locals.add(ValType::I32);
 
     let mut body = builder.func_body();
 
-    // Call previous start function if one is available. Currently this is
-    // always true because LLVM injects a call to `__wasm_init_memory` as the
-    // start function which, well, initializes memory.
-    if let Some(prev) = module.start.take() {
-        body.call(prev);
-    }
-
     let malloc = find_function(module, "__wbindgen_malloc")?;
 
     // Perform an if/else based on whether we're the first thread or not. Our
@@ -324,6 +341,7 @@ fn inject_start(
     body.i32_const(thread_counter_addr)
         .i32_const(1)
         .atomic_rmw(memory, AtomicOp::Add, AtomicWidth::I32, ATOMIC_MEM_ARG)
+        .local_tee(thread_count)
         .if_else(
             None,
             // If our thread id is nonzero then we're the second or greater thread, so
@@ -360,13 +378,7 @@ fn inject_start(
         .global_get(tls.base)
         .call(tls.init);
 
-    // Finish off our newly generated function.
-    let start_id = builder.finish(Vec::new(), &mut module.funcs);
-
-    // ... and finally flag it as the new start function
-    module.start = Some(start_id);
-
-    Ok(())
+    Ok(ThreadCount(thread_count))
 }
 
 fn inject_destroy(
diff --git a/crates/threads-xform/tests/all.rs b/crates/threads-xform/tests/all.rs
index 1101ccd2fcb..5bb7cb5fbc1 100644
--- a/crates/threads-xform/tests/all.rs
+++ b/crates/threads-xform/tests/all.rs
@@ -22,7 +22,7 @@ fn runtest(test: &Test) -> Result<String> {
 
     let config = wasm_bindgen_threads_xform::Config::new();
 
-    config.run(&mut module)?;
+    config.run(&mut module, &mut None)?;
     walrus::passes::gc::run(&mut module);
 
     let features = {