diff --git a/lib/dispatcher/env-http-proxy-agent.js b/lib/dispatcher/env-http-proxy-agent.js
index f88437f1936..9a602f465d4 100644
--- a/lib/dispatcher/env-http-proxy-agent.js
+++ b/lib/dispatcher/env-http-proxy-agent.js
@@ -10,6 +10,10 @@ const DEFAULT_PORTS = {
   'https:': 443
 }
 
+function normalizeProxyURI (proxy, defaultScheme = 'http://') {
+  return /^[a-zA-Z][a-zA-Z\d+.-]*:\/\//.test(proxy) ? proxy : `${defaultScheme}${proxy}`
+}
+
 class EnvHttpProxyAgent extends DispatcherBase {
   #noProxyValue = null
   #noProxyEntries = null
@@ -25,14 +29,14 @@ class EnvHttpProxyAgent extends DispatcherBase {
 
     const HTTP_PROXY = httpProxy ?? process.env.http_proxy ?? process.env.HTTP_PROXY
     if (HTTP_PROXY) {
-      this[kHttpProxyAgent] = new ProxyAgent({ ...agentOpts, uri: HTTP_PROXY })
+      this[kHttpProxyAgent] = new ProxyAgent({ ...agentOpts, uri: normalizeProxyURI(HTTP_PROXY, 'http://') })
     } else {
       this[kHttpProxyAgent] = this[kNoProxyAgent]
     }
 
     const HTTPS_PROXY = httpsProxy ?? process.env.https_proxy ?? process.env.HTTPS_PROXY
     if (HTTPS_PROXY) {
-      this[kHttpsProxyAgent] = new ProxyAgent({ ...agentOpts, uri: HTTPS_PROXY })
+      this[kHttpsProxyAgent] = new ProxyAgent({ ...agentOpts, uri: normalizeProxyURI(HTTPS_PROXY, 'https://') })
     } else {
       this[kHttpsProxyAgent] = this[kHttpProxyAgent]
     }
diff --git a/test/env-http-proxy-agent.js b/test/env-http-proxy-agent.js
index 97969cfaf20..483a5eec886 100644
--- a/test/env-http-proxy-agent.js
+++ b/test/env-http-proxy-agent.js
@@ -53,6 +53,16 @@ test('creates separate proxy agent for http and https when http_proxy and https_
   return dispatcher.close()
 })
 
+test('assumes protocol-specific scheme for schema-less proxy env vars', async (t) => {
+  t = tspl(t, { plan: 2 })
+  process.env.http_proxy = 'localhost:8080'
+  process.env.https_proxy = 'localhost:8443'
+  const dispatcher = new EnvHttpProxyAgent()
+  t.equal(dispatcher[kHttpProxyAgent][kProxy].uri, 'http://localhost:8080/')
+  t.equal(dispatcher[kHttpsProxyAgent][kProxy].uri, 'https://localhost:8443/')
+  return dispatcher.close()
+})
+
 test('handles uppercase HTTP_PROXY and HTTPS_PROXY', async (t) => {
   t = tspl(t, { plan: 6 })
   process.env.HTTP_PROXY = 'http://example.com:8080'