diff --git a/src/expand.rs b/src/expand.rs
index fae434e..785fc14 100644
--- a/src/expand.rs
+++ b/src/expand.rs
@@ -184,6 +184,12 @@ fn transform_sig(sig: &mut Signature, args: &RecursionArgs) {
         quote!()
     };
 
+    let sync_bound: TokenStream = if args.sync_bound {
+        quote!(+ ::core::marker::Sync)
+    } else {
+        quote!()
+    };
+
     let where_clause = sig
         .generics
         .where_clause
@@ -207,6 +213,6 @@ fn transform_sig(sig: &mut Signature, args: &RecursionArgs) {
     // Modify the return type
     sig.output = parse_quote! {
         -> ::core::pin::Pin<Box<
-            dyn ::core::future::Future<Output = #ret> #box_lifetime #send_bound >>
+            dyn ::core::future::Future<Output = #ret> #box_lifetime #send_bound #sync_bound >>
     };
 }
diff --git a/src/parse.rs b/src/parse.rs
index 728dcbf..0785402 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -22,30 +22,48 @@ impl Parse for AsyncItem {
 
 pub struct RecursionArgs {
     pub send_bound: bool,
+    pub sync_bound: bool,
 }
 
 impl Default for RecursionArgs {
     fn default() -> Self {
-        RecursionArgs { send_bound: true }
+        RecursionArgs {
+            send_bound: true,
+            sync_bound: false,
+        }
     }
 }
 
 /// Custom keywords for parser
 mod kw {
     syn::custom_keyword!(Send);
+    syn::custom_keyword!(Sync);
 }
 
 impl Parse for RecursionArgs {
     fn parse(input: ParseStream) -> Result<Self> {
+        let Self {
+            mut send_bound,
+            mut sync_bound,
+        } = Self::default();
         // Check for the `?Send` option
         if input.peek(Token![?]) {
             input.parse::<Question>()?;
             input.parse::<kw::Send>()?;
-            Ok(Self { send_bound: false })
-        } else if !input.is_empty() {
+            send_bound = false;
+        }
+        if input.peek(kw::Sync) {
+            input.parse::<kw::Sync>()?;
+            sync_bound = true;
+        }
+
+        if !input.is_empty() {
             Err(input.error("expected `?Send` or empty"))
         } else {
-            Ok(Self::default())
+            Ok(Self {
+                send_bound,
+                sync_bound,
+            })
         }
     }
 }
diff --git a/tests/sync.rs b/tests/sync.rs
new file mode 100644
index 0000000..31566b3
--- /dev/null
+++ b/tests/sync.rs
@@ -0,0 +1,21 @@
+use std::future::pending;
+
+use async_recursion::async_recursion;
+
+
+#[async_recursion(Sync)]
+async fn send_plus_sync() {
+    pending::<()>().await;
+
+}
+
+#[async_recursion(?Send Sync)]
+async fn only_sync() {
+    pending::<()>().await;
+}
+
+#[async_recursion(?Send)]
+async fn neither() {
+    pending::<()>().await;
+
+}
\ No newline at end of file