diff --git a/lib/config.js b/lib/config.js
index 1192afbac..4972587cd 100644
--- a/lib/config.js
+++ b/lib/config.js
@@ -411,34 +411,59 @@ function parseConfig (configFilePath, cliOptions, parseOptions) {
   // add the user's configuration in
   config.set(cliOptions)
 
+  let configModuleReturn
   try {
-    configModule(config)
+    configModuleReturn = configModule(config)
   } catch (e) {
     return fail('Error in config file!\n', e)
   }
+  function finalizeConfig (config) {
+    // merge the config from config file and cliOptions (precedence)
+    config.set(cliOptions)
 
-  // merge the config from config file and cliOptions (precedence)
-  config.set(cliOptions)
-
-  // if the user changed listenAddress, but didn't set a hostname, warn them
-  if (config.hostname === null && config.listenAddress !== null) {
-    log.warn(`ListenAddress was set to ${config.listenAddress} but hostname was left as the default: ` +
+    // if the user changed listenAddress, but didn't set a hostname, warn them
+    if (config.hostname === null && config.listenAddress !== null) {
+      log.warn(`ListenAddress was set to ${config.listenAddress} but hostname was left as the default: ` +
       `${defaultHostname}. If your browsers fail to connect, consider changing the hostname option.`)
-  }
-  // restore values that weren't overwritten by the user
-  if (config.hostname === null) {
-    config.hostname = defaultHostname
-  }
-  if (config.listenAddress === null) {
-    config.listenAddress = defaultListenAddress
-  }
+    }
+    // restore values that weren't overwritten by the user
+    if (config.hostname === null) {
+      config.hostname = defaultHostname
+    }
+    if (config.listenAddress === null) {
+      config.listenAddress = defaultListenAddress
+    }
 
-  // configure the logger as soon as we can
-  logger.setup(config.logLevel, config.colors, config.loggers)
+    // configure the logger as soon as we can
+    logger.setup(config.logLevel, config.colors, config.loggers)
 
-  log.debug(configFilePath ? `Loading config ${configFilePath}` : 'No config file specified.')
+    log.debug(configFilePath ? `Loading config ${configFilePath}` : 'No config file specified.')
 
-  return normalizeConfig(config, configFilePath)
+    return normalizeConfig(config, configFilePath)
+  }
+  const returnIsThenable = configModuleReturn != null && typeof configModuleReturn === 'object' && typeof configModuleReturn.then === 'function'
+  const promiseConfig = parseOptions && parseOptions.promiseConfig === true
+  if (returnIsThenable) {
+    if (promiseConfig !== true) {
+      const errorMessage =
+        'The `parseOptions.promiseConfig` option must be set to `true` to ' +
+        'enable promise return values from configuration files.'
+      return fail(errorMessage)
+    }
+    return configModuleReturn.then(function onKarmaConfigModuleFulfilled (/* ignoredResolutionValue */) {
+      return finalizeConfig(config)
+    })
+  } else {
+    if (promiseConfig) {
+      try {
+        return Promise.resolve(finalizeConfig(config))
+      } catch (exception) {
+        return Promise.reject(exception)
+      }
+    } else {
+      return finalizeConfig(config)
+    }
+  }
 }
 
 // PUBLIC API
diff --git a/lib/server.js b/lib/server.js
index a6ae81dab..bace88622 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -55,22 +55,36 @@ function createSocketIoServer (webServer, executor, config) {
 }
 
 class Server extends KarmaEventEmitter {
-  constructor (cliOptions, done) {
+  constructor (cliOptionsOrConfig, done) {
     super()
-    logger.setupFromConfig(cliOptions)
+    logger.setupFromConfig({
+      colors: cliOptionsOrConfig.colors,
+      logLevel: cliOptionsOrConfig.logLevel
+    })
 
     this.log = logger.create('karma-server')
 
     this.loadErrors = []
 
     let config
-    try {
-      config = cfg.parseConfig(cliOptions.configFile, cliOptions, { throwErrors: true })
-    } catch (parseConfigError) {
-      // TODO: change how `done` falls back to exit in next major version
-      //  SEE: https://github.com/karma-runner/karma/pull/3635#discussion_r565399378
-      (done || process.exit)(1)
-      return
+    if (cliOptionsOrConfig instanceof cfg.Config) {
+      config = cliOptionsOrConfig
+    } else {
+      try {
+        config = cfg.parseConfig(
+          cliOptionsOrConfig.configFile,
+          cliOptionsOrConfig,
+          {
+            promiseConfig: false,
+            throwErrors: true
+          }
+        )
+      } catch (parseConfigError) {
+        // TODO: change how `done` falls back to exit in next major version
+        //  SEE: https://github.com/karma-runner/karma/pull/3635#discussion_r565399378
+        (done || process.exit)(1)
+        return
+      }
     }
 
     this.log.debug('Final config', util.inspect(config, false, /** depth **/ null))