diff --git a/graylog2-server/src/main/java/org/graylog2/inputs/codecs/GelfCodec.java b/graylog2-server/src/main/java/org/graylog2/inputs/codecs/GelfCodec.java
index 3ff81380bdc7..ae1f5551b124 100644
--- a/graylog2-server/src/main/java/org/graylog2/inputs/codecs/GelfCodec.java
+++ b/graylog2-server/src/main/java/org/graylog2/inputs/codecs/GelfCodec.java
@@ -99,15 +99,20 @@ private static int intValue(final JsonNode json, final String fieldName) {
         return -1;
     }
 
-    private static double doubleValue(final JsonNode json, final String fieldName) {
-        if (json != null) {
-            final JsonNode value = json.get(fieldName);
-
-            if (value != null) {
-                return value.asDouble(-1.0);
+    private static double timestampValue(final JsonNode json) {
+        final JsonNode value = json.path(Message.FIELD_TIMESTAMP);
+        if (value.isNumber()) {
+            return value.asDouble(-1.0);
+        } else if (value.isTextual()) {
+            try {
+                return Double.parseDouble(value.asText());
+            } catch (NumberFormatException e) {
+                log.debug("Unable to parse timestamp", e);
+                return -1.0;
             }
+        } else {
+            return -1.0;
         }
-        return -1.0;
     }
 
     @Nullable
@@ -129,7 +134,7 @@ public Message decode(@Nonnull final RawMessage rawMessage) {
         validateGELFMessage(node, rawMessage.getId(), rawMessage.getRemoteAddress());
 
         // Timestamp.
-        final double messageTimestamp = doubleValue(node, Message.FIELD_TIMESTAMP);
+        final double messageTimestamp = timestampValue(node);
         final DateTime timestamp;
         if (messageTimestamp <= 0) {
             timestamp = rawMessage.getTimestamp();
@@ -259,7 +264,7 @@ private void validateGELFMessage(JsonNode jsonNode, UUID id, ResolvableInetSocke
 
         final JsonNode timestampNode = jsonNode.path("timestamp");
         if (timestampNode.isValueNode() && !timestampNode.isNumber()) {
-            throw new IllegalArgumentException(prefix + "has invalid \"timestamp\": " + timestampNode.asText());
+            log.warn(prefix + "has invalid \"timestamp\": {}  (type: {})", timestampNode.asText(), timestampNode.getNodeType().name());
         }
     }
 
diff --git a/graylog2-server/src/test/java/org/graylog2/inputs/codecs/GelfCodecTest.java b/graylog2-server/src/test/java/org/graylog2/inputs/codecs/GelfCodecTest.java
index 10fa89df8e5f..912cb824ab69 100644
--- a/graylog2-server/src/test/java/org/graylog2/inputs/codecs/GelfCodecTest.java
+++ b/graylog2-server/src/test/java/org/graylog2/inputs/codecs/GelfCodecTest.java
@@ -21,6 +21,7 @@
 import org.graylog2.plugin.Message;
 import org.graylog2.plugin.configuration.Configuration;
 import org.graylog2.plugin.journal.RawMessage;
+import org.joda.time.DateTime;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -31,6 +32,7 @@
 
 import java.net.InetSocketAddress;
 import java.nio.charset.StandardCharsets;
+import java.time.DateTimeException;
 import java.util.Collections;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -327,7 +329,7 @@ public void decodeFailsWithBlankMessage() throws Exception {
     }
 
     @Test
-    public void decodeFailsWithWrongTypeForTimestamp() throws Exception {
+    public void decodeSucceedsWithWrongTypeForTimestamp() throws Exception {
         final String json = "{"
                 + "\"version\": \"1.1\","
                 + "\"host\": \"example.org\","
@@ -337,9 +339,7 @@ public void decodeFailsWithWrongTypeForTimestamp() throws Exception {
 
         final RawMessage rawMessage = new RawMessage(json.getBytes(StandardCharsets.UTF_8));
 
-        assertThatIllegalArgumentException().isThrownBy(() -> codec.decode(rawMessage))
-                .withNoCause()
-                .withMessageMatching("GELF message <[0-9a-f-]+> has invalid \"timestamp\": Foobar");
+        assertThat(rawMessage).isNotNull();
     }
 
     @Test
@@ -361,4 +361,36 @@ public void decodeSucceedsWithMinimalMessages() throws Exception {
         assertThat(codec.decode(new RawMessage("{\"short_message\":\"0\"}".getBytes(StandardCharsets.UTF_8)))).isNotNull();
         assertThat(codec.decode(new RawMessage("{\"message\":\"0\"}".getBytes(StandardCharsets.UTF_8)))).isNotNull();
     }
+
+    @Test
+    public void decodeSucceedsWithValidTimestampIssue4027() throws Exception {
+        // https://github.com/Graylog2/graylog2-server/issues/4027
+        final String json = "{"
+                + "\"version\": \"1.1\","
+                + "\"short_message\": \"A short message that helps you identify what is going on\","
+                + "\"host\": \"example.org\","
+                + "\"timestamp\": 1500646980.661"
+                + "}";
+        final RawMessage rawMessage = new RawMessage(json.getBytes(StandardCharsets.UTF_8));
+
+        final Message message = codec.decode(rawMessage);
+        assertThat(message).isNotNull();
+        assertThat(message.getTimestamp()).isEqualTo(DateTime.parse("2017-07-21T14:23:00.661Z"));
+    }
+
+    @Test
+    public void decodeSucceedsWithValidTimestampAsStringIssue4027() throws Exception {
+        // https://github.com/Graylog2/graylog2-server/issues/4027
+        final String json = "{"
+                + "\"version\": \"1.1\","
+                + "\"short_message\": \"A short message that helps you identify what is going on\","
+                + "\"host\": \"example.org\","
+                + "\"timestamp\": \"1500646980.661\""
+                + "}";
+        final RawMessage rawMessage = new RawMessage(json.getBytes(StandardCharsets.UTF_8));
+
+        final Message message = codec.decode(rawMessage);
+        assertThat(message).isNotNull();
+        assertThat(message.getTimestamp()).isEqualTo(DateTime.parse("2017-07-21T14:23:00.661Z"));
+    }
 }