diff --git a/changelog/unreleased/SOLR-6973-signature-partial-update.yml b/changelog/unreleased/SOLR-6973-signature-partial-update.yml
new file mode 100644
index 000000000000..36cb04a0f974
--- /dev/null
+++ b/changelog/unreleased/SOLR-6973-signature-partial-update.yml
@@ -0,0 +1,7 @@
+title: SignatureUpdateProcessorFactory no longer adds an empty signature to (or deletes duplicates for) a partial update that contains none of the signature fields.
+type: fixed
+authors:
+  - name: Nick Shanin
+links:
+  - name: SOLR-6973
+    url: https://issues.apache.org/jira/browse/SOLR-6973
diff --git a/solr/core/src/java/org/apache/solr/update/processor/SignatureUpdateProcessorFactory.java b/solr/core/src/java/org/apache/solr/update/processor/SignatureUpdateProcessorFactory.java
index fd99601357db..e895bfb5d215 100644
--- a/solr/core/src/java/org/apache/solr/update/processor/SignatureUpdateProcessorFactory.java
+++ b/solr/core/src/java/org/apache/solr/update/processor/SignatureUpdateProcessorFactory.java
@@ -162,6 +162,22 @@ public void processAdd(AddUpdateCommand cmd) throws IOException {
           currDocSigFields = sigFields;
         }
 
+        if (isPartialUpdate) {
+          for (String field : currDocSigFields) {
+            if (doc.getField(field) != null) {
+              throw new SolrException(
+                  ErrorCode.SERVER_ERROR,
+                  "Can't use SignatureUpdateProcessor with partial update request "
+                      + "containing signature field: "
+                      + field);
+            }
+          }
+          // none of the signature fields are in the partial update, so there is nothing to sign;
+          // an empty signature would overwrite or delete unrelated documents
+          if (next != null) next.processAdd(cmd);
+          return;
+        }
+
         Signature sig =
             req.getCore().getResourceLoader().newInstance(signatureClass, Signature.class);
         sig.init(params);
@@ -169,13 +185,6 @@ public void processAdd(AddUpdateCommand cmd) throws IOException {
         for (String field : currDocSigFields) {
           SolrInputField f = doc.getField(field);
           if (f != null) {
-            if (isPartialUpdate) {
-              throw new SolrException(
-                  ErrorCode.SERVER_ERROR,
-                  "Can't use SignatureUpdateProcessor with partial update request "
-                      + "containing signature field: "
-                      + field);
-            }
             sig.add(field);
             Object o = f.getValue();
             if (o instanceof Collection) {
diff --git a/solr/core/src/test/org/apache/solr/update/processor/SignatureUpdateProcessorFactoryTest.java b/solr/core/src/test/org/apache/solr/update/processor/SignatureUpdateProcessorFactoryTest.java
index 05487931e063..1056f7527174 100644
--- a/solr/core/src/test/org/apache/solr/update/processor/SignatureUpdateProcessorFactoryTest.java
+++ b/solr/core/src/test/org/apache/solr/update/processor/SignatureUpdateProcessorFactoryTest.java
@@ -20,6 +20,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
 import org.apache.lucene.util.Constants;
 import org.apache.solr.SolrTestCaseJ4;
 import org.apache.solr.client.solrj.request.JavaBinRequestWriter;
@@ -34,6 +35,7 @@
 import org.apache.solr.request.SolrQueryRequest;
 import org.apache.solr.request.SolrQueryRequestBase;
 import org.apache.solr.response.SolrQueryResponse;
+import org.apache.solr.update.AddUpdateCommand;
 import org.junit.Before;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -200,6 +202,39 @@ public void run() {
     checkNumDocs(1);
   }
 
+  @Test
+  public void testPartialUpdateWithoutSignatureFieldsIsPassedThrough() throws Exception {
+    SignatureUpdateProcessorFactory factory = new SignatureUpdateProcessorFactory();
+    NamedList<Object> args = new NamedList<>();
+    args.add("signatureField", "signatureField");
+    args.add("overwriteDupes", true);
+    args.add("fields", "v_t,t_field");
+    args.add("signatureClass", "org.apache.solr.update.processor.Lookup3Signature");
+    factory.init(args);
+
+    AtomicReference<AddUpdateCommand> seen = new AtomicReference<>();
+    try (SolrQueryRequest r = req()) {
+      UpdateRequestProcessor proc =
+          factory.getInstance(
+              r,
+              new SolrQueryResponse(),
+              new UpdateRequestProcessor(null) {
+                @Override
+                public void processAdd(AddUpdateCommand cmd) {
+                  seen.set(cmd);
+                }
+              });
+      AddUpdateCommand cmd = new AddUpdateCommand(r);
+      cmd.solrDoc = new SolrInputDocument();
+      cmd.solrDoc.addField("id", "1a");
+      cmd.solrDoc.addField("name", Map.of("set", "changed"));
+      proc.processAdd(cmd);
+    }
+    assertNotNull(seen.get());
+    assertNull(seen.get().solrDoc.getField("signatureField"));
+    assertNull(seen.get().updateTerm);
+  }
+
   /** a non-indexed signatureField is fine as long as overwriteDupes==false */
   @Test
   public void testNonIndexedSignatureField() throws Exception {