diff --git a/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/PropertiesFileTransformerTest.kt b/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/PropertiesFileTransformerTest.kt
new file mode 100644
index 000000000..a050dd1f5
--- /dev/null
+++ b/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/PropertiesFileTransformerTest.kt
@@ -0,0 +1,164 @@
+package com.github.jengelman.gradle.plugins.shadow.transformers
+
+import assertk.assertThat
+import assertk.assertions.contains
+import assertk.assertions.isEqualTo
+import com.github.jengelman.gradle.plugins.shadow.transformers.PropertiesFileTransformer.MergeStrategy
+import com.github.jengelman.gradle.plugins.shadow.util.Issue
+import com.github.jengelman.gradle.plugins.shadow.util.getContent
+import kotlin.io.path.appendText
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.EnumSource
+
+class PropertiesFileTransformerTest : BaseTransformerTest() {
+
+  @ParameterizedTest
+  @EnumSource(MergeStrategy::class)
+  fun mergePropertiesWithDifferentStrategies(strategy: MergeStrategy) {
+    val one = buildJarOne {
+      insert("META-INF/test.properties", "key1=one\nkey2=one")
+    }
+    val two = buildJarTwo {
+      insert("META-INF/test.properties", "key2=two\nkey3=two")
+    }
+    projectScriptPath.appendText(
+      transform<PropertiesFileTransformer>(
+        dependenciesBlock = implementationFiles(one, two),
+        transformerBlock = """
+          mergeStrategy = $mergeStrategyClassName.$strategy
+          mergeSeparator = ";"
+          paths = ["META-INF/test.properties"]
+        """.trimIndent(),
+      ),
+    )
+
+    run(shadowJarTask)
+
+    val expected = when (strategy) {
+      MergeStrategy.First -> arrayOf("key1=one", "key2=one", "key3=two")
+      MergeStrategy.Latest -> arrayOf("key1=one", "key2=two", "key3=two")
+      MergeStrategy.Append -> arrayOf("key1=one", "key2=one;two", "key3=two")
+    }
+    val content = outputShadowJar.use { it.getContent("META-INF/test.properties") }
+    assertThat(content).contains(*expected)
+  }
+
+  @Test
+  fun mergePropertiesWithKeyTransformer() {
+    val one = buildJarOne {
+      insert("META-INF/test.properties", "foo=bar")
+    }
+    val two = buildJarTwo {
+      insert("META-INF/test.properties", "FOO=baz")
+    }
+    projectScriptPath.appendText(
+      transform<PropertiesFileTransformer>(
+        dependenciesBlock = implementationFiles(one, two),
+        transformerBlock = """
+          mergeStrategy = $mergeStrategyClassName.Append
+          keyTransformer = { key -> key.toUpperCase() }
+          paths = ["META-INF/test.properties"]
+        """.trimIndent(),
+      ),
+    )
+
+    run(shadowJarTask)
+
+    val content = outputShadowJar.use { it.getContent("META-INF/test.properties") }
+    assertThat(content).contains("FOO=bar,baz")
+  }
+
+  @Test
+  fun mergePropertiesWithSpecifiedCharset() {
+    val one = buildJarOne {
+      insert("META-INF/utf8.properties", "foo=第一")
+    }
+    val two = buildJarTwo {
+      insert("META-INF/utf8.properties", "foo=第二")
+    }
+    projectScriptPath.appendText(
+      transform<PropertiesFileTransformer>(
+        dependenciesBlock = implementationFiles(one, two),
+        transformerBlock = """
+          mergeStrategy = $mergeStrategyClassName.Append
+                charsetName = "utf-8"
+                paths = ["META-INF/utf8.properties"]
+        """.trimIndent(),
+      ),
+    )
+
+    run(shadowJarTask)
+
+    val content = outputShadowJar.use { it.getContent("META-INF/utf8.properties") }
+    assertThat(content).contains("foo=第一,第二")
+  }
+
+  @Test
+  fun mergePropertiesWithMappings() {
+    val one = buildJarOne {
+      insert("META-INF/foo.properties", "foo=1")
+      insert("META-INF/bar.properties", "bar=2")
+    }
+    val two = buildJarTwo {
+      insert("META-INF/foo.properties", "foo=3")
+      insert("META-INF/bar.properties", "bar=4")
+    }
+    projectScriptPath.appendText(
+      transform<PropertiesFileTransformer>(
+        dependenciesBlock = implementationFiles(one, two),
+        transformerBlock = """
+          mappings = [
+            "META-INF/foo.properties": ["mergeStrategy": "append", "mergeSeparator": ";"],
+            "META-INF/bar.properties": ["mergeStrategy": "latest"]
+          ]
+        """.trimIndent(),
+      ),
+    )
+
+    run(shadowJarTask)
+
+    assertThat(outputShadowJar).useAll {
+      getContent("META-INF/foo.properties").contains("foo=1;3")
+      getContent("META-INF/bar.properties").contains("bar=4")
+    }
+  }
+
+  @Issue(
+    "https://github.com/GradleUp/shadow/issues/622",
+    "https://github.com/GradleUp/shadow/issues/856",
+  )
+  @Test
+  fun mergedPropertiesDontContainDateComment() {
+    val one = buildJarOne {
+      insert("META-INF/test.properties", "foo=one")
+    }
+    val two = buildJarTwo {
+      insert("META-INF/test.properties", "foo=two")
+    }
+    projectScriptPath.appendText(
+      transform<PropertiesFileTransformer>(
+        dependenciesBlock = implementationFiles(one, two),
+        transformerBlock = """
+          mergeStrategy = $mergeStrategyClassName.Append
+          paths = ["META-INF/test.properties"]
+        """.trimIndent(),
+      ),
+    )
+
+    run(shadowJarTask)
+
+    val content = outputShadowJar.use { it.getContent("META-INF/test.properties") }
+    assertThat(content.trimIndent()).isEqualTo(
+      """
+        #
+
+        foo=one,two
+      """.trimIndent(),
+    )
+  }
+
+  private companion object {
+    val mergeStrategyClassName = requireNotNull(MergeStrategy::class.java.canonicalName)
+  }
+}
diff --git a/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/TransformersTest.kt b/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/TransformersTest.kt
index 2ed0f9019..6940adad5 100644
--- a/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/TransformersTest.kt
+++ b/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/TransformersTest.kt
@@ -195,7 +195,6 @@ class TransformersTest : BaseTransformerTest() {
       "{ resource.set(\"test.file\"); file.fileValue(file(\"test/some.file\")) }" to IncludeResourceTransformer::class,
       "" to ManifestAppenderTransformer::class,
       "" to ManifestResourceTransformer::class,
-      "{ keyTransformer = { it.toLowerCase() } }" to PropertiesFileTransformer::class,
     )
   }
 }
diff --git a/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/CleanProperties.kt b/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/CleanProperties.kt
index 13f6730f7..624aa422d 100644
--- a/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/CleanProperties.kt
+++ b/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/CleanProperties.kt
@@ -6,6 +6,9 @@ import java.io.Writer
 import java.util.Date
 import java.util.Properties
 
+/**
+ * Introduced in order to remove prepended timestamp when creating output stream.
+ */
 internal class CleanProperties : Properties() {
   @Throws(IOException::class)
   override fun store(writer: Writer, comments: String) {