diff --git a/lib/core/constants.js b/lib/core/constants.js
new file mode 100644
index 00000000000..0f827cc4ae0
--- /dev/null
+++ b/lib/core/constants.js
@@ -0,0 +1,116 @@
+/** @type {Record<string, string | undefined>} */
+const headerNameLowerCasedRecord = {}
+
+// https://developer.mozilla.org/docs/Web/HTTP/Headers
+const wellknownHeaderNames = [
+  'Accept',
+  'Accept-Encoding',
+  'Accept-Language',
+  'Accept-Ranges',
+  'Access-Control-Allow-Credentials',
+  'Access-Control-Allow-Headers',
+  'Access-Control-Allow-Methods',
+  'Access-Control-Allow-Origin',
+  'Access-Control-Expose-Headers',
+  'Access-Control-Max-Age',
+  'Access-Control-Request-Headers',
+  'Access-Control-Request-Method',
+  'Age',
+  'Allow',
+  'Alt-Svc',
+  'Alt-Used',
+  'Authorization',
+  'Cache-Control',
+  'Clear-Site-Data',
+  'Connection',
+  'Content-Disposition',
+  'Content-Encoding',
+  'Content-Language',
+  'Content-Length',
+  'Content-Location',
+  'Content-Range',
+  'Content-Security-Policy',
+  'Content-Security-Policy-Report-Only',
+  'Content-Type',
+  'Cookie',
+  'Cross-Origin-Embedder-Policy',
+  'Cross-Origin-Opener-Policy',
+  'Cross-Origin-Resource-Policy',
+  'Date',
+  'Device-Memory',
+  'Downlink',
+  'ECT',
+  'ETag',
+  'Expect',
+  'Expect-CT',
+  'Expires',
+  'Forwarded',
+  'From',
+  'Host',
+  'If-Match',
+  'If-Modified-Since',
+  'If-None-Match',
+  'If-Range',
+  'If-Unmodified-Since',
+  'Keep-Alive',
+  'Last-Modified',
+  'Link',
+  'Location',
+  'Max-Forwards',
+  'Origin',
+  'Permissions-Policy',
+  'Pragma',
+  'Proxy-Authenticate',
+  'Proxy-Authorization',
+  'RTT',
+  'Range',
+  'Referer',
+  'Referrer-Policy',
+  'Refresh',
+  'Retry-After',
+  'Sec-WebSocket-Accept',
+  'Sec-WebSocket-Extensions',
+  'Sec-WebSocket-Key',
+  'Sec-WebSocket-Protocol',
+  'Sec-WebSocket-Version',
+  'Server',
+  'Server-Timing',
+  'Service-Worker-Allowed',
+  'Service-Worker-Navigation-Preload',
+  'Set-Cookie',
+  'SourceMap',
+  'Strict-Transport-Security',
+  'Supports-Loading-Mode',
+  'TE',
+  'Timing-Allow-Origin',
+  'Trailer',
+  'Transfer-Encoding',
+  'Upgrade',
+  'Upgrade-Insecure-Requests',
+  'User-Agent',
+  'Vary',
+  'Via',
+  'WWW-Authenticate',
+  'X-Content-Type-Options',
+  'X-DNS-Prefetch-Control',
+  'X-Frame-Options',
+  'X-Permitted-Cross-Domain-Policies',
+  'X-Powered-By',
+  'X-Requested-With',
+  'X-XSS-Protection'
+]
+
+for (let i = 0; i < wellknownHeaderNames.length; ++i) {
+  const key = wellknownHeaderNames[i]
+  const lowerCasedKey = key.toLowerCase()
+  headerNameLowerCasedRecord[key] = headerNameLowerCasedRecord[lowerCasedKey] =
+    lowerCasedKey
+}
+
+// Note: object prototypes should not be able to be referenced. e.g. `Object#hasOwnProperty`.
+Object.setPrototypeOf(headerNameLowerCasedRecord, null)
+
+module.exports = {
+  wellknownHeaderNames,
+  headerNameLowerCasedRecord
+}
diff --git a/lib/core/util.js b/lib/core/util.js
index 82cf3260b9f..5a28529b663 100644
--- a/lib/core/util.js
+++ b/lib/core/util.js
@@ -9,6 +9,7 @@ const { InvalidArgumentError } = require('./errors')
 const { Blob } = require('buffer')
 const nodeUtil = require('util')
 const { stringify } = require('querystring')
+const { headerNameLowerCasedRecord } = require('./constants')
 
 const [nodeMajor, nodeMinor] = process.versions.node.split('.').map(v => Number(v))
 
@@ -223,19 +224,21 @@ function parseHeaders (headers, obj = {}) {
   if (!Array.isArray(headers)) return headers
 
   for (let i = 0; i < headers.length; i += 2) {
-    const key = headers[i].toString().toLowerCase()
-    let val = obj[key]
+    const key = headers[i].toString()
+    const lowerCasedKey = headerNameLowerCasedRecord[key] ?? key.toLowerCase()
+    let val = obj[lowerCasedKey]
 
     if (!val) {
-      if (Array.isArray(headers[i + 1])) {
-        obj[key] = headers[i + 1].map(x => x.toString('utf8'))
+      const headersValue = headers[i + 1]
+      if (typeof headersValue === 'string') {
+        obj[lowerCasedKey] = headersValue
       } else {
-        obj[key] = headers[i + 1].toString('utf8')
+        obj[lowerCasedKey] = Array.isArray(headersValue) ? headersValue.map(x => x.toString('utf8')) : headersValue.toString('utf8')
       }
     } else {
       if (!Array.isArray(val)) {
         val = [val]
-        obj[key] = val
+        obj[lowerCasedKey] = val
       }
       val.push(headers[i + 1].toString('utf8'))
     }
diff --git a/test/util.js b/test/util.js
index cd5c4ed3044..75a4d8c1617 100644
--- a/test/util.js
+++ b/test/util.js
@@ -6,6 +6,7 @@ const { Stream } = require('stream')
 const { EventEmitter } = require('events')
 
 const util = require('../lib/core/util')
+const { headerNameLowerCasedRecord } = require('../lib/core/constants')
 const { InvalidArgumentError } = require('../lib/core/errors')
 
 test('isStream', (t) => {
@@ -121,3 +122,8 @@ test('buildURL', (t) => {
 
   t.end()
 })
+
+test('headerNameLowerCasedRecord', (t) => {
+  t.plan(1)
+  t.ok(typeof headerNameLowerCasedRecord.hasOwnProperty === 'undefined')
+})