diff --git a/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/DeprecationChecks.java b/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/DeprecationChecks.java
index a2d222e33db39..4ba5e5a2b58e2 100644
--- a/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/DeprecationChecks.java
+++ b/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/DeprecationChecks.java
@@ -57,7 +57,9 @@ private DeprecationChecks() {
             IndexDeprecationChecks::percolatorUnmappedFieldsAsStringCheck,
             IndexDeprecationChecks::indexNameCheck,
 			IndexDeprecationChecks::nodeLeftDelayedTimeCheck,
-            IndexDeprecationChecks::shardOnStartupCheck
+            IndexDeprecationChecks::shardOnStartupCheck,
+            IndexDeprecationChecks::classicSimilarityMappingCheck,
+            IndexDeprecationChecks::classicSimilaritySettingsCheck
             ));
 
     /**
diff --git a/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecks.java b/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecks.java
index bfef54cf62394..b6397db8b9055 100644
--- a/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecks.java
+++ b/x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecks.java
@@ -24,6 +24,7 @@
 import java.util.Map;
 import java.util.function.BiConsumer;
 import java.util.function.Function;
+import java.util.stream.Collectors;
 
 /**
  * Index-specific deprecation checks
@@ -133,7 +134,38 @@ static DeprecationIssue percolatorUnmappedFieldsAsStringCheck(IndexMetaData inde
         }
         return null;
     }
-	
+
+    static DeprecationIssue classicSimilarityMappingCheck(IndexMetaData indexMetaData) {
+        List<String> issues = new ArrayList<>();
+        fieldLevelMappingIssue(indexMetaData, ((mappingMetaData, sourceAsMap) -> issues.addAll(
+            findInPropertiesRecursively(mappingMetaData.type(), sourceAsMap,
+                property -> "classic".equals(property.get("similarity"))))));
+        if (issues.size() > 0) {
+            return new DeprecationIssue(DeprecationIssue.Level.WARNING,
+                "Classic similarity has been removed",
+                "https://www.elastic.co/guide/en/elasticsearch/reference/master/" +
+                    "#_the_literal_classic_literal_similarity_has_been_removed",
+                "Fields which use classic similarity: " + issues.toString());
+        }
+        return null;
+    }
+
+    static DeprecationIssue classicSimilaritySettingsCheck(IndexMetaData indexMetaData) {
+        Map<String, Settings> similarities = indexMetaData.getSettings().getGroups("index.similarity");
+        List<String> classicSimilarities = similarities.entrySet().stream()
+            .filter(entry -> "classic".equals(entry.getValue().get("type")))
+            .map(Map.Entry::getKey)
+            .collect(Collectors.toList());
+        if (classicSimilarities.size() > 0) {
+            return new DeprecationIssue(DeprecationIssue.Level.WARNING,
+                "Classic similarity has been removed",
+                "https://www.elastic.co/guide/en/elasticsearch/reference/master/" +
+                    "#_the_literal_classic_literal_similarity_has_been_removed",
+                "Custom similarities defined using classic similarity: " + classicSimilarities.toString());
+        }
+        return null;
+    }
+
 	 static DeprecationIssue nodeLeftDelayedTimeCheck(IndexMetaData indexMetaData) {
         String setting = UnassignedInfo.INDEX_DELAYED_NODE_LEFT_TIMEOUT_SETTING.getKey();
         String value = indexMetaData.getSettings().get(setting);
@@ -150,7 +182,7 @@ static DeprecationIssue nodeLeftDelayedTimeCheck(IndexMetaData indexMetaData) {
         }
         return null;
     }
-	
+
 	static DeprecationIssue shardOnStartupCheck(IndexMetaData indexMetaData) {
         String setting = IndexSettings.INDEX_CHECK_ON_STARTUP.getKey();
         String value = indexMetaData.getSettings().get(setting);
diff --git a/x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecksTests.java b/x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecksTests.java
index 5935e50b3d1cc..82c1954816fe9 100644
--- a/x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecksTests.java
+++ b/x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecksTests.java
@@ -15,6 +15,7 @@
 import org.elasticsearch.xpack.core.deprecation.DeprecationInfoAction;
 import org.elasticsearch.xpack.core.deprecation.DeprecationIssue;
 
+import java.io.IOException;
 import java.util.List;
 import java.util.Locale;
 
@@ -109,6 +110,53 @@ public void testPercolatorUnmappedFieldsAsStringCheck() {
         assertTrue(noIssues.isEmpty());
     }
 
+    public void testClassicSimilarityMappingCheck() throws IOException {
+        String mappingJson = "{\n" +
+            "  \"properties\": {\n" +
+            "    \"default_field\": {\n" +
+            "      \"type\": \"text\"\n" +
+            "    },\n" +
+            "    \"classic_sim_field\": {\n" +
+            "      \"type\": \"text\",\n" +
+            "      \"similarity\": \"classic\"\n" +
+            "    }\n" +
+            "  }\n" +
+            "}";
+        IndexMetaData index = IndexMetaData.builder(randomAlphaOfLengthBetween(5,10))
+            .settings(settings(
+                VersionUtils.randomVersionBetween(random(), Version.V_6_0_0, VersionUtils.getPreviousVersion(Version.CURRENT))))
+            .numberOfShards(randomIntBetween(1,100))
+            .numberOfReplicas(randomIntBetween(1, 100))
+            .putMapping("_doc", mappingJson)
+            .build();
+        DeprecationIssue expected = new DeprecationIssue(DeprecationIssue.Level.WARNING,
+            "Classic similarity has been removed",
+            "https://www.elastic.co/guide/en/elasticsearch/reference/master/" +
+                "#_the_literal_classic_literal_similarity_has_been_removed",
+            "Fields which use classic similarity: [[type: _doc, field: classic_sim_field]]");
+        List<DeprecationIssue> issues = DeprecationChecks.filterChecks(INDEX_SETTINGS_CHECKS, c -> c.apply(index));
+        assertEquals(singletonList(expected), issues);
+    }
+
+    public void testClassicSimilaritySettingsCheck() {
+        IndexMetaData index = IndexMetaData.builder(randomAlphaOfLengthBetween(5, 10))
+            .settings(settings(
+                VersionUtils.randomVersionBetween(random(), Version.V_6_0_0, VersionUtils.getPreviousVersion(Version.CURRENT)))
+                .put("index.similarity.my_classic_similarity.type", "classic")
+                .put("index.similarity.my_okay_similarity.type", "BM25"))
+            .numberOfShards(randomIntBetween(1, 100))
+            .numberOfReplicas(randomIntBetween(1, 100))
+            .build();
+
+        DeprecationIssue expected = new DeprecationIssue(DeprecationIssue.Level.WARNING,
+            "Classic similarity has been removed",
+            "https://www.elastic.co/guide/en/elasticsearch/reference/master/" +
+                "#_the_literal_classic_literal_similarity_has_been_removed",
+            "Custom similarities defined using classic similarity: [my_classic_similarity]");
+        List<DeprecationIssue> issues = DeprecationChecks.filterChecks(INDEX_SETTINGS_CHECKS, c -> c.apply(index));
+        assertEquals(singletonList(expected), issues);
+    }
+
     public void testNodeLeftDelayedTimeCheck() {
         String negativeTimeValue = "-" + randomPositiveTimeValue();
         String indexName = randomAlphaOfLengthBetween(0, 10);