diff --git a/config/src/main/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDsl.kt b/config/src/main/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDsl.kt
index 57260b3d96..b663f4d717 100644
--- a/config/src/main/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDsl.kt
+++ b/config/src/main/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDsl.kt
@@ -16,6 +16,7 @@
 
 package org.springframework.security.config.web.server
 
+import org.springframework.security.authentication.ReactiveAuthenticationManager
 import org.springframework.security.oauth2.client.registration.ReactiveClientRegistrationRepository
 import org.springframework.security.web.server.SecurityWebFilterChain
 import org.springframework.security.web.server.util.matcher.ServerWebExchangeMatcher
@@ -57,10 +58,13 @@ operator fun ServerHttpSecurity.invoke(httpConfiguration: ServerHttpSecurityDsl.
  * @author Eleftheria Stein
  * @since 5.4
  * @param init the configurations to apply to the provided [ServerHttpSecurity]
+ * @property authenticationManager the default [ReactiveAuthenticationManager] to use
  */
 @ServerSecurityMarker
 class ServerHttpSecurityDsl(private val http: ServerHttpSecurity, private val init: ServerHttpSecurityDsl.() -> Unit) {
 
+    var authenticationManager: ReactiveAuthenticationManager? = null
+
     /**
      * Allows configuring the [ServerHttpSecurity] to only be invoked when matching the
      * provided [ServerWebExchangeMatcher].
@@ -630,6 +634,7 @@ class ServerHttpSecurityDsl(private val http: ServerHttpSecurity, private val in
      */
     internal fun build(): SecurityWebFilterChain {
         init()
+        authenticationManager?.also { this.http.authenticationManager(authenticationManager) }
         return this.http.build()
     }
 }
diff --git a/config/src/test/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDslTests.kt b/config/src/test/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDslTests.kt
index 2128c9c9d4..2b8361b71c 100644
--- a/config/src/test/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDslTests.kt
+++ b/config/src/test/kotlin/org/springframework/security/config/web/server/ServerHttpSecurityDslTests.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,9 @@
 
 package org.springframework.security.config.web.server
 
+import io.mockk.every
+import io.mockk.mockkObject
+import io.mockk.verify
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.Rule
 import org.junit.Test
@@ -23,6 +26,8 @@ import org.springframework.beans.factory.annotation.Autowired
 import org.springframework.context.ApplicationContext
 import org.springframework.context.annotation.Bean
 import org.springframework.http.HttpHeaders
+import org.springframework.security.authentication.ReactiveAuthenticationManager
+import org.springframework.security.authentication.TestingAuthenticationToken
 import org.springframework.security.config.annotation.web.reactive.EnableWebFluxSecurity
 import org.springframework.security.config.test.SpringTestRule
 import org.springframework.security.web.header.writers.frameoptions.XFrameOptionsHeaderWriter
@@ -199,4 +204,36 @@ class ServerHttpSecurityDslTests {
     class CustomWebFilter : WebFilter {
         override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> = Mono.empty()
     }
+
+    @Test
+    fun `authentication manager when configured in DSL then used`() {
+        this.spring.register(AuthenticationManagerConfig::class.java).autowire()
+        mockkObject(AuthenticationManagerConfig.AUTHENTICATION_MANAGER)
+        every {
+            AuthenticationManagerConfig.AUTHENTICATION_MANAGER.authenticate(any())
+        } returns Mono.just(TestingAuthenticationToken("user", "password", "ROLE_USER"))
+        this.client.get().uri("/").headers { headers ->
+            headers.setBasicAuth("user", "password")
+        }.exchange()
+        verify(exactly = 1) { AuthenticationManagerConfig.AUTHENTICATION_MANAGER.authenticate(any()) }
+    }
+
+    @EnableWebFlux
+    @EnableWebFluxSecurity
+    open class AuthenticationManagerConfig {
+        companion object {
+            val AUTHENTICATION_MANAGER: ReactiveAuthenticationManager = ReactiveAuthenticationManager { Mono.empty() }
+        }
+
+        @Bean
+        open fun springWebFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
+            return http {
+                authenticationManager = AUTHENTICATION_MANAGER
+                authorizeExchange {
+                    authorize(anyExchange, authenticated)
+                }
+                httpBasic { }
+            }
+        }
+    }
 }