diff --git a/src/main/java/org/apache/commons/collections4/map/CaseInsensitiveMap.java b/src/main/java/org/apache/commons/collections4/map/CaseInsensitiveMap.java
index e1e7daf011..a6238be41d 100644
--- a/src/main/java/org/apache/commons/collections4/map/CaseInsensitiveMap.java
+++ b/src/main/java/org/apache/commons/collections4/map/CaseInsensitiveMap.java
@@ -141,6 +141,63 @@ protected Object convertKey(final Object key) {
         return AbstractHashedMap.NULL;
     }
 
+    @Override
+    public V put(final K key, final V value) {
+        final Object convertedKey = convertKey(key);
+        final int hashCode = hash(convertedKey);
+        final int index = hashIndex(hashCode, data.length);
+        HashEntry<K, V> entry = data[index];
+        while (entry != null) {
+            if (entry.hashCode == hashCode && isEqualKey(convertedKey, entry.key)) {
+                final V oldValue = entry.getValue();
+                updateEntry(entry, value);
+                return oldValue;
+            }
+            entry = entry.next;
+        }
+
+        addMappingWithoutKeyConversion(index, hashCode, convertedKey, value);
+        return null;
+    }
+
+    /**
+     * Adds a new key-value mapping into this map.
+     * <p>
+     * This implementation calls {@code createEntryWithoutKeyConversion()}, {@code addEntry()}
+     * and {@code checkCapacity()}.
+     * It also handles changes to {@code modCount} and {@code size}.
+     * Subclasses could override to fully control adds to the map.
+     *
+     * @param hashIndex  the index into the data array to store at
+     * @param hashCode  the hash code of the key to add
+     * @param convertedKey  the key to add, already converted to lower case by {@code convertKey()}
+     * @param value  the value to add
+     */
+    protected void addMappingWithoutKeyConversion(final int hashIndex, final int hashCode, final Object convertedKey, final V value) {
+        modCount++;
+        final HashEntry<K, V> entry = createEntryWithoutKeyConversion(data[hashIndex], hashCode, convertedKey, value);
+        addEntry(entry, hashIndex);
+        size++;
+        checkCapacity();
+    }
+
+    /**
+     * Creates an entry to store the key-value data.
+     * <p>
+     * This implementation creates a new HashEntry instance.
+     * Subclasses can override this to return a different storage class,
+     * or implement caching.
+     *
+     * @param next  the next entry in sequence
+     * @param hashCode  the hash code to use
+     * @param convertedKey  the key to add, already converted to lower case by {@code convertKey()}
+     * @param value  the value to store
+     * @return the newly created entry
+     */
+    protected HashEntry<K, V> createEntryWithoutKeyConversion(final HashEntry<K, V> next, final int hashCode, final Object convertedKey, final V value) {
+        return new HashEntry<>(next, hashCode, convertedKey, value);
+    }
+
     /**
      * Clones the map without cloning the keys or values.
      *
diff --git a/src/test/java/org/apache/commons/collections4/map/CaseInsensitiveMapTest.java b/src/test/java/org/apache/commons/collections4/map/CaseInsensitiveMapTest.java
index 13d503b3e2..a635b00ba8 100644
--- a/src/test/java/org/apache/commons/collections4/map/CaseInsensitiveMapTest.java
+++ b/src/test/java/org/apache/commons/collections4/map/CaseInsensitiveMapTest.java
@@ -21,6 +21,7 @@
 import java.util.Map;
 import java.util.Set;
 
+import org.easymock.EasyMock;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -147,4 +148,20 @@ public void testPutAll() {
             || !caseInsensitiveMap.containsValue("Three")); // ones collapsed
         assertEquals("Four", caseInsensitiveMap.get(null));
     }
+
+    // COLLECTIONS-803
+    @Test
+    @SuppressWarnings("unchecked")
+    public void testPutConvertKeyOnlyOnce() {
+        CaseInsensitiveMap<String, String> mock = EasyMock.partialMockBuilder(CaseInsensitiveMap.class)
+                .addMockedMethod("convertKey")
+                .createMock();
+        mock.data = new AbstractHashedMap.HashEntry[16];
+        EasyMock.expect(mock.convertKey("Key")).andReturn("key").once();
+        EasyMock.replay(mock);
+
+        mock.put("Key", "value");
+
+        EasyMock.verify(mock);
+    }
 }