diff --git a/plugins/http2_adapter/CHANGELOG.md b/plugins/http2_adapter/CHANGELOG.md
index a65d22591..585f347b1 100644
--- a/plugins/http2_adapter/CHANGELOG.md
+++ b/plugins/http2_adapter/CHANGELOG.md
@@ -5,6 +5,7 @@
 - Fix cancellation for streamed responses and downloads.
 - Fix progress for streamed responses and downloads.
 - Bump minimum Dart SDK to 3.0.0 as required by the `http2` package.
+- Allows `HTTP/1.0` when connecting to proxies.
 
 ## 2.4.0
 
diff --git a/plugins/http2_adapter/lib/src/connection_manager.dart b/plugins/http2_adapter/lib/src/connection_manager.dart
index 8dc8ae5f4..b97bfe823 100644
--- a/plugins/http2_adapter/lib/src/connection_manager.dart
+++ b/plugins/http2_adapter/lib/src/connection_manager.dart
@@ -1,16 +1,18 @@
 part of 'http2_adapter.dart';
 
-/// ConnectionManager is used to manager the connections that should be reusable.
-/// The main responsibility of ConnectionManager is to implement a connection reuse
-/// strategy for http2.
+/// Manages the connections that should be reusable.
+/// It implements a connection reuse strategy for HTTP/2.
 abstract class ConnectionManager {
   factory ConnectionManager({
     Duration idleTimeout = const Duration(seconds: 15),
     void Function(Uri uri, ClientSetting)? onClientCreate,
+    ProxyConnectedPredicate proxyConnectedPredicate =
+        defaultProxyConnectedPredicate,
   }) =>
       _ConnectionManager(
         idleTimeout: idleTimeout,
         onClientCreate: onClientCreate,
+        proxyConnectedPredicate: proxyConnectedPredicate,
       );
 
   /// Get the connection(may reuse) for each request.
@@ -20,3 +22,13 @@ abstract class ConnectionManager {
 
   void close({bool force = false});
 }
+
+/// {@template dio_http2_adapter.ProxyConnectedPredicate}
+/// Checks whether the proxy has been connected through the given [status].
+/// {@endtemplate}
+typedef ProxyConnectedPredicate = bool Function(String protocol, String status);
+
+/// Accepts HTTP/1.x connections for proxies.
+bool defaultProxyConnectedPredicate(String protocol, String status) {
+  return status.startsWith(RegExp(r'HTTP/1+\.\d 200'));
+}
diff --git a/plugins/http2_adapter/lib/src/connection_manager_imp.dart b/plugins/http2_adapter/lib/src/connection_manager_imp.dart
index 036039dee..28430b8fb 100644
--- a/plugins/http2_adapter/lib/src/connection_manager_imp.dart
+++ b/plugins/http2_adapter/lib/src/connection_manager_imp.dart
@@ -5,6 +5,7 @@ class _ConnectionManager implements ConnectionManager {
   _ConnectionManager({
     Duration? idleTimeout,
     this.onClientCreate,
+    this.proxyConnectedPredicate = defaultProxyConnectedPredicate,
   }) : _idleTimeout = idleTimeout ?? const Duration(seconds: 1);
 
   /// Callback when socket created.
@@ -13,6 +14,9 @@ class _ConnectionManager implements ConnectionManager {
   /// for unverifiable certificates.
   final void Function(Uri uri, ClientSetting)? onClientCreate;
 
+  /// {@macro dio_http2_adapter.ProxyConnectedPredicate}
+  final ProxyConnectedPredicate proxyConnectedPredicate;
+
   /// Sets the idle timeout(milliseconds) of non-active persistent
   /// connections. For the sake of socket reuse feature with http/2,
   /// the value should not be less than 1 second.
@@ -173,7 +177,9 @@ class _ConnectionManager implements ConnectionManager {
     // Use CRLF as the end of the line https://www.ietf.org/rfc/rfc2616.txt
     const crlf = '\r\n';
 
-    proxySocket.write('CONNECT ${target.host}:${target.port} HTTP/1.1');
+    // TODO(EVERYONE): Figure out why we can only use an HTTP/1.x proxy here.
+    const proxyProtocol = 'HTTP/1.1';
+    proxySocket.write('CONNECT ${target.host}:${target.port} $proxyProtocol');
     proxySocket.write(crlf);
     proxySocket.write('Host: ${target.host}:${target.port}');
 
@@ -203,16 +209,24 @@ class _ConnectionManager implements ConnectionManager {
         final response = ascii.decode(event);
         final lines = response.split(crlf);
         final statusLine = lines.first;
-
-        if (statusLine.startsWith('HTTP/1.1 200')) {
-          completerProxyInitialization.complete();
-        } else {
-          completerProxyInitialization.completeError(
-            SocketException('Proxy cannot be initialized'),
-          );
+        if (!completerProxyInitialization.isCompleted) {
+          if (proxyConnectedPredicate(proxyProtocol, statusLine)) {
+            completerProxyInitialization.complete();
+          } else {
+            completerProxyInitialization.completeError(
+              SocketException(
+                'Proxy cannot be initialized with status = [$statusLine], '
+                'host = ${target.host}, port = ${target.port}',
+              ),
+            );
+          }
+        }
+      },
+      onError: (e, s) {
+        if (!completerProxyInitialization.isCompleted) {
+          completerProxyInitialization.completeError(e, s);
         }
       },
-      onError: completerProxyInitialization.completeError,
     );
 
     await completerProxyInitialization.future;
diff --git a/plugins/http2_adapter/test/http2_test.dart b/plugins/http2_adapter/test/http2_test.dart
index 41bee3e56..319b40cd4 100644
--- a/plugins/http2_adapter/test/http2_test.dart
+++ b/plugins/http2_adapter/test/http2_test.dart
@@ -161,4 +161,22 @@ void main() {
     expect(content, contains('Numkey: 2'));
     expect(content, contains('Booleankey: false'));
   });
+
+  group(ProxyConnectedPredicate, () {
+    group('defaultProxyConnectedPredicate', () {
+      test(
+        'accepts HTTP/1.x for HTTP/1.1 proxy',
+        () {
+          expect(
+            defaultProxyConnectedPredicate('HTTP/1.1', 'HTTP/1.1 200'),
+            true,
+          );
+          expect(
+            defaultProxyConnectedPredicate('HTTP/1.1', 'HTTP/1.0 200'),
+            true,
+          );
+        },
+      );
+    });
+  });
 }