diff --git a/Sources/Base/OAuth2Base.swift b/Sources/Base/OAuth2Base.swift
index 70db5ea9..42477267 100644
--- a/Sources/Base/OAuth2Base.swift
+++ b/Sources/Base/OAuth2Base.swift
@@ -417,12 +417,15 @@ open class OAuth2Base: OAuth2Securable {
 	This method checks `state`, throws `OAuth2Error.missingState` or `OAuth2Error.invalidState`. Resets state if it matches.
 	*/
 	public final func assureMatchesState(_ params: OAuth2JSON) throws {
-		guard let state = params["state"] as? String, !state.isEmpty else {
-			throw OAuth2Error.missingState
-		}
-		logger?.trace("OAuth2", msg: "Checking state, got “\(state)”, expecting “\(context.state)”")
-		if !context.matchesState(state) {
-			throw OAuth2Error.invalidState
+		if let state = params["state"] as? String, !state.isEmpty {
+			logger?.trace("OAuth2", msg: "Checking state, got “\(state)”, expecting “\(context.state)”")
+			if !context.matchesState(state) {
+				throw OAuth2Error.invalidState
+			}
+		} else {
+			if !clientConfig.stateParameterOptional {
+				throw OAuth2Error.missingState
+			}
 		}
 		context.resetState()
 	}
diff --git a/Sources/Base/OAuth2ClientConfig.swift b/Sources/Base/OAuth2ClientConfig.swift
index 2a128875..668b0e68 100644
--- a/Sources/Base/OAuth2ClientConfig.swift
+++ b/Sources/Base/OAuth2ClientConfig.swift
@@ -71,6 +71,9 @@ open class OAuth2ClientConfig {
 	
 	/// Add custom parameters to the authorization request.
 	public var customParameters: [String: String]? = nil
+
+	/// Whether the state parameter is optional
+	public var stateParameterOptional = false
 	
 	/// Most servers use UTF-8 encoding for Authorization headers, but that's not 100% true: make it configurable (see https://github.com/p2/OAuth2/issues/165).
 	open var authStringEncoding = String.Encoding.utf8
@@ -134,7 +137,9 @@ open class OAuth2ClientConfig {
 		if let params = settings["parameters"] as? OAuth2StringDict {
 			customParameters = params
 		}
-		
+		if let stateOptional = settings["state_parameter_optional"] as? Bool {
+			stateParameterOptional = stateOptional
+		}
 		// access token options
 		if let assume = settings["token_assume_unexpired"] as? Bool {
 			accessTokenAssumeUnexpired = assume
diff --git a/Tests/FlowTests/OAuth2CodeGrantTests.swift b/Tests/FlowTests/OAuth2CodeGrantTests.swift
index c656de8f..7fd298ca 100644
--- a/Tests/FlowTests/OAuth2CodeGrantTests.swift
+++ b/Tests/FlowTests/OAuth2CodeGrantTests.swift
@@ -179,6 +179,28 @@ class OAuth2CodeGrantTests: XCTestCase {
 			XCTAssertTrue(false, "Should not throw, but threw \(error)")
 		}
 	}
+
+	func testRedirectURINoStateParameterAllowed() {
+		let settings: OAuth2JSON = [
+			"client_id": "abc",
+			"client_secret": "xyz",
+			"authorize_uri": "https://auth.ful.io",
+			"token_uri": "https://token.ful.io",
+			"keychain": false,
+			"state_parameter_optional": true
+		]
+		let oauth = OAuth2CodeGrant(settings: settings)
+		oauth.redirect = "oauth2://callback"
+		oauth.context.redirectURL = oauth.redirect
+		// parse no state
+		let redirect = URL(string: "oauth2://callback?code=C0D3")!
+		do {
+			_ = try oauth.validateRedirectURL(redirect)
+		}
+		catch let error {
+			XCTAssertTrue(false, "Must not end up here with \(error)")
+		}
+	}
 	
 	func testTokenRequest() {
 		let oauth = OAuth2CodeGrant(settings: [