diff --git a/spring-boot-test/pom.xml b/spring-boot-test/pom.xml
index 47a6313996..f4b28b8a93 100644
--- a/spring-boot-test/pom.xml
+++ b/spring-boot-test/pom.xml
@@ -122,6 +122,11 @@
 			<artifactId>spring-boot-test-support</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>ch.qos.logback</groupId>
+			<artifactId>logback-classic</artifactId>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.apache.tomcat.embed</groupId>
 			<artifactId>tomcat-embed-core</artifactId>
@@ -143,6 +148,16 @@
 			<artifactId>kotlin-runtime</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.slf4j</groupId>
+			<artifactId>jcl-over-slf4j</artifactId>
+			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>org.slf4j</groupId>
+			<artifactId>slf4j-api</artifactId>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.spockframework</groupId>
 			<artifactId>spock-core</artifactId>
diff --git a/spring-boot-test/src/main/java/org/springframework/boot/test/json/DuplicateJsonObjectContextCustomizerFactory.java b/spring-boot-test/src/main/java/org/springframework/boot/test/json/DuplicateJsonObjectContextCustomizerFactory.java
new file mode 100644
index 0000000000..d3846ab240
--- /dev/null
+++ b/spring-boot-test/src/main/java/org/springframework/boot/test/json/DuplicateJsonObjectContextCustomizerFactory.java
@@ -0,0 +1,90 @@
+/*
+ * Copyright 2012-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.test.json;
+
+import java.net.URL;
+import java.util.ArrayList;
+import java.util.Enumeration;
+import java.util.List;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.test.context.ContextConfigurationAttributes;
+import org.springframework.test.context.ContextCustomizer;
+import org.springframework.test.context.ContextCustomizerFactory;
+import org.springframework.test.context.MergedContextConfiguration;
+
+/**
+ * A {@link ContextCustomizerFactory} that produces a {@link ContextCustomizer} that warns
+ * the user when multiple occurrences of {@code JSONObject} are found on the class path.
+ *
+ * @author Andy Wilkinson
+ */
+class DuplicateJsonObjectContextCustomizerFactory implements ContextCustomizerFactory {
+
+	@Override
+	public ContextCustomizer createContextCustomizer(Class<?> testClass,
+			List<ContextConfigurationAttributes> configAttributes) {
+		return new DuplicateJsonObjectContextCustomizer();
+	}
+
+	private static class DuplicateJsonObjectContextCustomizer
+			implements ContextCustomizer {
+
+		private final Log logger = LogFactory
+				.getLog(DuplicateJsonObjectContextCustomizer.class);
+
+		@Override
+		public void customizeContext(ConfigurableApplicationContext context,
+				MergedContextConfiguration mergedConfig) {
+			List<URL> jsonObjects = findJsonObjects();
+			if (jsonObjects.size() > 1) {
+				logDuplicateJsonObjectsWarning(jsonObjects);
+			}
+		}
+
+		private List<URL> findJsonObjects() {
+			List<URL> jsonObjects = new ArrayList<URL>();
+			try {
+				Enumeration<URL> resources = getClass().getClassLoader()
+						.getResources("org/json/JSONObject.class");
+				while (resources.hasMoreElements()) {
+					jsonObjects.add(resources.nextElement());
+				}
+			}
+			catch (Exception ex) {
+				// Continue
+			}
+			return jsonObjects;
+		}
+
+		private void logDuplicateJsonObjectsWarning(List<URL> jsonObjects) {
+			StringBuilder message = new StringBuilder("\n\nFound multiple occurrences of"
+					+ " org.json.JSONObject on the class path:\n\n");
+			for (URL jsonObject : jsonObjects) {
+				message.append("\t" + jsonObject + "\n");
+			}
+			message.append("\nYou may wish to exclude one of them to ensure"
+					+ " predictable runtime behaviour\n");
+			this.logger.warn(message);
+		}
+
+	}
+
+}
diff --git a/spring-boot-test/src/main/resources/META-INF/spring.factories b/spring-boot-test/src/main/resources/META-INF/spring.factories
index 3a9340c3bc..96e7abd05c 100644
--- a/spring-boot-test/src/main/resources/META-INF/spring.factories
+++ b/spring-boot-test/src/main/resources/META-INF/spring.factories
@@ -3,6 +3,7 @@ org.springframework.test.context.ContextCustomizerFactory=\
 org.springframework.boot.test.context.ImportsContextCustomizerFactory,\
 org.springframework.boot.test.context.SpringBootTestContextCustomizerFactory,\
 org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizerFactory,\
+org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory,\
 org.springframework.boot.test.mock.mockito.MockitoContextCustomizerFactory,\
 org.springframework.boot.test.web.reactive.WebTestClientContextCustomizerFactory
 
diff --git a/spring-boot-test/src/test/java/org/springframework/boot/test/json/DuplicateJsonObjectContextCustomizerFactoryTests.java b/spring-boot-test/src/test/java/org/springframework/boot/test/json/DuplicateJsonObjectContextCustomizerFactoryTests.java
new file mode 100644
index 0000000000..53e5f4ed66
--- /dev/null
+++ b/spring-boot-test/src/test/java/org/springframework/boot/test/json/DuplicateJsonObjectContextCustomizerFactoryTests.java
@@ -0,0 +1,49 @@
+/*
+ * Copyright 2012-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.test.json;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.junit.runner.classpath.ClassPathOverrides;
+import org.springframework.boot.junit.runner.classpath.ModifiedClassPathRunner;
+import org.springframework.boot.test.rule.OutputCapture;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link DuplicateJsonObjectContextCustomizerFactory}.
+ *
+ * @author Andy Wilkinson
+ */
+@RunWith(ModifiedClassPathRunner.class)
+@ClassPathOverrides("org.json:json:20140107")
+public class DuplicateJsonObjectContextCustomizerFactoryTests {
+
+	@Rule
+	public OutputCapture output = new OutputCapture();
+
+	@Test
+	public void warningForMultipleVersions() {
+		new DuplicateJsonObjectContextCustomizerFactory()
+				.createContextCustomizer(null, null).customizeContext(null, null);
+		assertThat(this.output.toString()).contains(
+				"Found multiple occurrences of org.json.JSONObject on the class path:");
+	}
+
+}