diff --git a/spring-beans/src/main/java/org/springframework/beans/BeanUtils.java b/spring-beans/src/main/java/org/springframework/beans/BeanUtils.java
index 5a46e8d08a..053393e6c4 100644
--- a/spring-beans/src/main/java/org/springframework/beans/BeanUtils.java
+++ b/spring-beans/src/main/java/org/springframework/beans/BeanUtils.java
@@ -776,7 +776,14 @@ public abstract class BeanUtils {
 					if (readMethod != null) {
 						ResolvableType sourceResolvableType = ResolvableType.forMethodReturnType(readMethod);
 						ResolvableType targetResolvableType = ResolvableType.forMethodParameter(writeMethod, 0);
-						if (targetResolvableType.isAssignableFrom(sourceResolvableType)) {
+
+						// Ignore generic types in assignable check if either ResolvableType has unresolvable generics.
+						boolean isAssignable =
+								(sourceResolvableType.hasUnresolvableGenerics() || targetResolvableType.hasUnresolvableGenerics() ?
+										ClassUtils.isAssignable(writeMethod.getParameterTypes()[0], readMethod.getReturnType()) :
+										targetResolvableType.isAssignableFrom(sourceResolvableType));
+
+						if (isAssignable) {
 							try {
 								if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers())) {
 									readMethod.setAccessible(true);
diff --git a/spring-beans/src/test/java/org/springframework/beans/BeanUtilsTests.java b/spring-beans/src/test/java/org/springframework/beans/BeanUtilsTests.java
index ef1bbf4616..f3e9d2eaa9 100644
--- a/spring-beans/src/test/java/org/springframework/beans/BeanUtilsTests.java
+++ b/spring-beans/src/test/java/org/springframework/beans/BeanUtilsTests.java
@@ -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.
@@ -19,12 +19,16 @@ package org.springframework.beans;
 import java.beans.Introspector;
 import java.beans.PropertyDescriptor;
 import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationHandler;
+import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import java.lang.reflect.Proxy;
 import java.net.URI;
 import java.net.URL;
 import java.time.DayOfWeek;
 import java.time.LocalDateTime;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Date;
 import java.util.List;
 import java.util.Locale;
@@ -196,6 +200,29 @@ class BeanUtilsTests {
 		assertThat(longListHolder.getList()).isEmpty();
 	}
 
+	@Test  // gh-26531
+	void copyPropertiesIgnoresGenericsIfSourceOrTargetHasUnresolvableGenerics() throws Exception {
+		Order original = new Order("test", Arrays.asList("foo", "bar"));
+
+		// Create a Proxy that loses the generic type information for the getLineItems() method.
+		OrderSummary proxy = proxyOrder(original);
+		assertThat(OrderSummary.class.getDeclaredMethod("getLineItems").toGenericString())
+			.contains("java.util.List<java.lang.String>");
+		assertThat(proxy.getClass().getDeclaredMethod("getLineItems").toGenericString())
+			.contains("java.util.List")
+			.doesNotContain("<java.lang.String>");
+
+		// Ensure that our custom Proxy works as expected.
+		assertThat(proxy.getId()).isEqualTo("test");
+		assertThat(proxy.getLineItems()).containsExactly("foo", "bar");
+
+		// Copy from proxy to target.
+		Order target = new Order();
+		BeanUtils.copyProperties(proxy, target);
+		assertThat(target.getId()).isEqualTo("test");
+		assertThat(target.getLineItems()).containsExactly("foo", "bar");
+	}
+
 	@Test
 	void copyPropertiesWithEditable() throws Exception {
 		TestBean tb = new TestBean();
@@ -633,4 +660,77 @@ class BeanUtilsTests {
 		}
 	}
 
+	@SuppressWarnings("unused")
+	private static class Order {
+
+		private String id;
+		private List<String> lineItems;
+
+
+		Order() {
+		}
+
+		Order(String id, List<String> lineItems) {
+			this.id = id;
+			this.lineItems = lineItems;
+		}
+
+		public String getId() {
+			return id;
+		}
+
+		public void setId(String id) {
+			this.id = id;
+		}
+
+		public List<String> getLineItems() {
+			return this.lineItems;
+		}
+
+		public void setLineItems(List<String> lineItems) {
+			this.lineItems = lineItems;
+		}
+
+		@Override
+		public String toString() {
+			return "Order [id=" + this.id + ", lineItems=" + this.lineItems + "]";
+		}
+	}
+
+	private interface OrderSummary {
+
+		String getId();
+
+		List<String> getLineItems();
+	}
+
+
+	private OrderSummary proxyOrder(Order order) {
+		return (OrderSummary) Proxy.newProxyInstance(getClass().getClassLoader(),
+			new Class<?>[] { OrderSummary.class }, new OrderInvocationHandler(order));
+	}
+
+
+	private static class OrderInvocationHandler implements InvocationHandler {
+
+		private final Order order;
+
+
+		OrderInvocationHandler(Order order) {
+			this.order = order;
+		}
+
+		@Override
+		public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
+			try {
+				// Ignore args since OrderSummary doesn't declare any methods with arguments,
+				// and we're not supporting equals(Object), etc.
+				return Order.class.getDeclaredMethod(method.getName()).invoke(this.order);
+			}
+			catch (InvocationTargetException ex) {
+				throw ex.getTargetException();
+			}
+		}
+	}
+
 }