diff --git a/core/src/main/java/org/apache/calcite/sql/type/JavaToSqlTypeConversionRules.java b/core/src/main/java/org/apache/calcite/sql/type/JavaToSqlTypeConversionRules.java
index f1941b29fef1..7b4b8a2a8779 100644
--- a/core/src/main/java/org/apache/calcite/sql/type/JavaToSqlTypeConversionRules.java
+++ b/core/src/main/java/org/apache/calcite/sql/type/JavaToSqlTypeConversionRules.java
@@ -17,6 +17,7 @@
 package org.apache.calcite.sql.type;
 
 import org.apache.calcite.avatica.util.ArrayImpl;
+import org.apache.calcite.avatica.util.ByteString;
 
 import com.google.common.collect.ImmutableMap;
 
@@ -62,6 +63,7 @@ public class JavaToSqlTypeConversionRules {
           .put(boolean.class, SqlTypeName.BOOLEAN)
           .put(Boolean.class, SqlTypeName.BOOLEAN)
           .put(byte[].class, SqlTypeName.VARBINARY)
+          .put(ByteString.class, SqlTypeName.VARBINARY)
           .put(String.class, SqlTypeName.VARCHAR)
           .put(char[].class, SqlTypeName.VARCHAR)
           .put(Character.class, SqlTypeName.CHAR)
diff --git a/core/src/test/java/org/apache/calcite/sql/type/SqlTypeUtilTest.java b/core/src/test/java/org/apache/calcite/sql/type/SqlTypeUtilTest.java
index ac0bda1b87eb..37cd26aca6f9 100644
--- a/core/src/test/java/org/apache/calcite/sql/type/SqlTypeUtilTest.java
+++ b/core/src/test/java/org/apache/calcite/sql/type/SqlTypeUtilTest.java
@@ -16,6 +16,7 @@
  */
 package org.apache.calcite.sql.type;
 
+import org.apache.calcite.avatica.util.ByteString;
 import org.apache.calcite.rel.type.RelDataType;
 import org.apache.calcite.rel.type.RelDataTypeFactory;
 import org.apache.calcite.sql.SqlBasicTypeNameSpec;
@@ -293,4 +294,10 @@ private static void assertCanCast(RelDataType toType, RelDataType fromType) {
             "Expected to be able to cast from %s to %s without coercion.", fromType, toType),
         SqlTypeUtil.canCastFrom(toType, fromType, /* coerce= */ defaultRules), is(true));
   }
+
+  @Test void testJavaToSqlByteStringMapping() {
+    SqlTypeName sqlTypeName = JavaToSqlTypeConversionRules.instance().lookup(ByteString.class);
+    assertThat("ByteString.class should map to SqlTypeName.VARBINARY",
+        sqlTypeName, is(SqlTypeName.VARBINARY));
+  }
 }
diff --git a/core/src/test/java/org/apache/calcite/test/UdfTest.java b/core/src/test/java/org/apache/calcite/test/UdfTest.java
index 674b1e6c2aad..df70a1d185b1 100644
--- a/core/src/test/java/org/apache/calcite/test/UdfTest.java
+++ b/core/src/test/java/org/apache/calcite/test/UdfTest.java
@@ -182,6 +182,12 @@ private CalciteAssert.AssertThat withUdf() {
         + Smalls.AllTypesFunction.class.getName()
         + "',\n"
         + "           methodName: '*'\n"
+        + "         },\n"
+        + "         {\n"
+        + "           name: 'UNBASE64',\n"
+        + "           className: '"
+        + Smalls.MyUnbase64Function.class.getName()
+        + "'\n"
         + "         }\n"
         + "       ]\n"
         + "     }\n"
@@ -1075,6 +1081,22 @@ private static CalciteAssert.AssertThat withBadUdf(Class<?> clazz) {
     }
   }
 
+  /** Test case for
+   * <a href="https://issues.apache.org/jira/browse/CALCITE-7073">[CALCITE-7073] </a>
+   * Tests that the UNBASE64 user-defined function correctly decodes a Base64 string
+   * and its return type (VARBINARY, mapped from ByteString) is fully
+   * compatible for direct comparison with SQL VARBINARY literals (X'...')
+   * within queries. */
+  @Test void testUnbase64DirectComparison() {
+    final String testHex = "74657374"; // "test" in bytes
+    final String testBase64 = "dGVzdA=="; // Base64 for "test"
+
+    final String sql = "select \"adhoc\".unbase64(cast('" + testBase64 + "' as varchar))"
+        + " = x'" + testHex + "' as C\n";
+
+    withUdf().query(sql).returns("C=true\n");
+  }
+
   /**
    * Base class for functions that append arrays.
    */
diff --git a/testkit/src/main/java/org/apache/calcite/util/Smalls.java b/testkit/src/main/java/org/apache/calcite/util/Smalls.java
index 963b11590f66..3ffd3efda98d 100644
--- a/testkit/src/main/java/org/apache/calcite/util/Smalls.java
+++ b/testkit/src/main/java/org/apache/calcite/util/Smalls.java
@@ -19,6 +19,7 @@
 import org.apache.calcite.DataContext;
 import org.apache.calcite.adapter.enumerable.EnumerableTableScan;
 import org.apache.calcite.adapter.java.AbstractQueryableTable;
+import org.apache.calcite.avatica.util.ByteString;
 import org.apache.calcite.config.CalciteConnectionConfig;
 import org.apache.calcite.linq4j.AbstractEnumerable;
 import org.apache.calcite.linq4j.BaseQueryable;
@@ -1477,4 +1478,18 @@ private Object[] convertRow(Object[] full) {
           BuiltInMethod.QUERYABLE_AS_ENUMERABLE.method);
     }
   }
+
+  /** User-defined function that decodes a Base64 string to bytes. */
+  public static class MyUnbase64Function {
+    public static ByteString eval(String s) {
+      if (s == null) {
+        return null;
+      }
+      try {
+        return ByteString.ofBase64(s);
+      } catch (Exception e) {
+        return null;
+      }
+    }
+  }
 }