diff --git a/CHANGELOG.md b/CHANGELOG.md
index 176b2348e..383ee16c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@ The minor version will be incremented upon a breaking change and the patch versi
 - ts: Extract Anchor error codes into their own package ([#2983](https://github.com/coral-xyz/anchor/pull/2983)).
 - cli: Add additional solana arguments to the `upgrade` command ([#2998](https://github.com/coral-xyz/anchor/pull/2998)).
 - spl: Export `spl-associated-token-account` crate ([#2999](https://github.com/coral-xyz/anchor/pull/2999)).
+- lang: Support legacy IDLs with `declare_program!` ([#2997](https://github.com/coral-xyz/anchor/pull/2997)).
 
 ### Fixes
 
diff --git a/lang/attribute/program/Cargo.toml b/lang/attribute/program/Cargo.toml
index 3bd2a3dad..2d70f3bb5 100644
--- a/lang/attribute/program/Cargo.toml
+++ b/lang/attribute/program/Cargo.toml
@@ -16,7 +16,7 @@ idl-build = ["anchor-syn/idl-build"]
 interface-instructions = ["anchor-syn/interface-instructions"]
 
 [dependencies]
-anchor-lang-idl = { path = "../../../idl", version = "0.1.0" }
+anchor-lang-idl = { path = "../../../idl", version = "0.1.0", features = ["convert"] }
 anchor-syn = { path = "../../syn", version = "0.30.0" }
 anyhow = "1"
 bs58 = "0.5"
diff --git a/lang/attribute/program/src/declare_program/mod.rs b/lang/attribute/program/src/declare_program/mod.rs
index bca2c15ec..b6e5df3f2 100644
--- a/lang/attribute/program/src/declare_program/mod.rs
+++ b/lang/attribute/program/src/declare_program/mod.rs
@@ -35,19 +35,17 @@ impl ToTokens for DeclareProgram {
 
 fn get_idl(name: &syn::Ident) -> anyhow::Result<Idl> {
     let manifest_dir = std::env::var("CARGO_MANIFEST_DIR").expect("Failed to get manifest dir");
-    let path = std::path::Path::new(&manifest_dir)
+    std::path::Path::new(&manifest_dir)
         .ancestors()
         .find_map(|ancestor| {
             let idl_dir = ancestor.join("idls");
-            std::fs::metadata(&idl_dir).map(|_| idl_dir).ok()
+            idl_dir.exists().then_some(idl_dir)
         })
         .ok_or_else(|| anyhow!("`idls` directory not found"))
-        .map(|idl_dir| idl_dir.join(name.to_string()).with_extension("json"))?;
-
-    std::fs::read(path)
-        .map_err(|e| anyhow!("Failed to read IDL: {e}"))
-        .map(|idl| serde_json::from_slice(&idl))?
-        .map_err(|e| anyhow!("Failed to parse IDL: {e}"))
+        .map(|idl_dir| idl_dir.join(name.to_string()).with_extension("json"))
+        .map(std::fs::read)?
+        .map_err(|e| anyhow!("Failed to read IDL `{name}`: {e}"))
+        .map(|buf| Idl::from_slice_with_conversion(&buf))?
 }
 
 fn gen_program(idl: &Idl, name: &syn::Ident) -> proc_macro2::TokenStream {
diff --git a/tests/declare-program/idls/external_legacy.json b/tests/declare-program/idls/external_legacy.json
new file mode 100644
index 000000000..20da7a7fa
--- /dev/null
+++ b/tests/declare-program/idls/external_legacy.json
@@ -0,0 +1,65 @@
+{
+  "version": "0.1.0",
+  "name": "external",
+  "metadata": {
+    "address": "Externa111111111111111111111111111111111111"
+  },
+  "instructions": [
+    {
+      "name": "init",
+      "accounts": [
+        { "name": "authority", "isMut": true, "isSigner": true },
+        { "name": "myAccount", "isMut": true, "isSigner": false },
+        { "name": "systemProgram", "isMut": false, "isSigner": false }
+      ],
+      "args": []
+    },
+    {
+      "name": "update",
+      "accounts": [
+        { "name": "authority", "isMut": false, "isSigner": true },
+        { "name": "myAccount", "isMut": true, "isSigner": false }
+      ],
+      "args": [{ "name": "value", "type": "u32" }]
+    },
+    {
+      "name": "updateComposite",
+      "accounts": [
+        {
+          "name": "update",
+          "accounts": [
+            { "name": "authority", "isMut": false, "isSigner": true },
+            { "name": "myAccount", "isMut": true, "isSigner": false }
+          ]
+        }
+      ],
+      "args": [{ "name": "value", "type": "u32" }]
+    },
+    {
+      "name": "testCompilationDefinedTypeParam",
+      "accounts": [{ "name": "signer", "isMut": false, "isSigner": true }],
+      "args": [{ "name": "myAccount", "type": { "defined": "MyAccount" } }]
+    },
+    {
+      "name": "testCompilationReturnType",
+      "accounts": [{ "name": "signer", "isMut": false, "isSigner": true }],
+      "args": [],
+      "returns": "bool"
+    }
+  ],
+  "accounts": [
+    {
+      "name": "MyAccount",
+      "type": {
+        "kind": "struct",
+        "fields": [{ "name": "field", "type": "u32" }]
+      }
+    }
+  ],
+  "events": [
+    {
+      "name": "MyEvent",
+      "fields": [{ "name": "value", "type": "u32", "index": false }]
+    }
+  ]
+}
diff --git a/tests/declare-program/programs/declare-program/src/lib.rs b/tests/declare-program/programs/declare-program/src/lib.rs
index 5272dea55..01df3b22a 100644
--- a/tests/declare-program/programs/declare-program/src/lib.rs
+++ b/tests/declare-program/programs/declare-program/src/lib.rs
@@ -5,6 +5,9 @@ declare_id!("Dec1areProgram11111111111111111111111111111");
 declare_program!(external);
 use external::program::External;
 
+// Compilation check for legacy IDL (pre Anchor `0.30`)
+declare_program!(external_legacy);
+
 #[program]
 pub mod declare_program {
     use super::*;