diff --git a/transpiler/javatests/com/google/j2cl/transpiler/backend/kotlin/source/BUILD b/transpiler/javatests/com/google/j2cl/transpiler/backend/kotlin/source/BUILD
new file mode 100644
index 0000000000..6a82f5ac49
--- /dev/null
+++ b/transpiler/javatests/com/google/j2cl/transpiler/backend/kotlin/source/BUILD
@@ -0,0 +1,26 @@
+load("//testing/build_defs:junit_test_suites.bzl", "junit_test_suites")
+load("@io_bazel_rules_kotlin//kotlin:kotlin.bzl", "kt_jvm_library")
+
+package(
+    default_applicable_licenses = ["//:j2cl_license"],
+    licenses = ["notice"],
+)
+
+kt_jvm_library(
+    name = "lib",
+    testonly = 1,
+    srcs = [
+        "SourceTest.kt",
+    ],
+    deps = [
+        "//third_party/java/junit",
+        "//third_party/java/truth",
+        "//transpiler/java/com/google/j2cl/transpiler/backend/kotlin/source",
+    ],
+)
+
+junit_test_suites(
+    name = "junit",
+    sizes = ["small"],
+    deps = [":lib"],
+)
diff --git a/transpiler/javatests/com/google/j2cl/transpiler/backend/kotlin/source/SourceTest.kt b/transpiler/javatests/com/google/j2cl/transpiler/backend/kotlin/source/SourceTest.kt
new file mode 100644
index 0000000000..ed3749fc3c
--- /dev/null
+++ b/transpiler/javatests/com/google/j2cl/transpiler/backend/kotlin/source/SourceTest.kt
@@ -0,0 +1,163 @@
+/*
+ * Copyright 2023 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.j2cl.transpiler.backend.kotlin.source
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+@RunWith(JUnit4::class)
+class SourceTest {
+  private val sourceA: Source
+    get() = source("a")
+
+  private val sourceB: Source
+    get() = source("b")
+
+  private val sourceC: Source
+    get() = source("c")
+
+  private val emptySources: Iterable<Source>
+    get() = listOf()
+
+  private val sourcesA: Iterable<Source>
+    get() = listOf(sourceA)
+
+  private val sourcesAB: Iterable<Source>
+    get() = listOf(sourceA, sourceB)
+
+  private val sourcesABC: Iterable<Source>
+    get() = listOf(sourceA, sourceB, sourceC)
+
+  private val sourcesABCAndEmpty: Iterable<Source>
+    get() = listOf(sourceA, emptySource, sourceB, emptySource, sourceC)
+
+  private infix fun Source.gives(string: String) {
+    assertThat(toString()).isEqualTo(string)
+  }
+
+  @Test
+  fun emptySource() {
+    emptySource gives ""
+  }
+
+  @Test
+  fun stringSource() {
+    source("foo") gives "foo"
+  }
+
+  @Test
+  fun sourcePlusSource() {
+    sourceA + sourceB gives "ab"
+  }
+
+  @Test
+  fun joinSources() {
+    join(sourceA) gives "a"
+    join(sourceA, sourceB) gives "ab"
+    join(sourceA, emptySource, sourceB) gives "ab"
+  }
+
+  @Test
+  fun stringSeparatedSources() {
+    ", " separated emptySources gives ""
+    ", " separated sourcesA gives "a"
+    ", " separated sourcesAB gives "a, b"
+    ", " separated sourcesABC gives "a, b, c"
+    ", " separated sourcesABCAndEmpty gives "a, b, c"
+  }
+
+  @Test
+  fun separatedSources_vararg() {
+    spaceSeparated(sourceA, emptySource, sourceB, emptySource, sourceC) gives "a b c"
+    commaSeparated(sourceA, emptySource, sourceB, emptySource, sourceC) gives "a, b, c"
+    dotSeparated(sourceA, emptySource, sourceB, emptySource, sourceC) gives "a.b.c"
+    colonSeparated(sourceA, emptySource, sourceB, emptySource, sourceC) gives "a: b: c"
+    newLineSeparated(sourceA, emptySource, sourceB, emptySource, sourceC) gives "a\nb\nc"
+    emptyLineSeparated(sourceA, emptySource, sourceB, emptySource, sourceC) gives "a\n\nb\n\nc"
+  }
+
+  @Test
+  fun separatedSources_iterable() {
+    spaceSeparated(sourcesABCAndEmpty) gives "a b c"
+    commaSeparated(sourcesABCAndEmpty) gives "a, b, c"
+    commaAndNewLineSeparated(sourcesABCAndEmpty) gives "a,\nb,\nc"
+    dotSeparated(sourcesABCAndEmpty) gives "a.b.c"
+    ampersandSeparated(sourcesABCAndEmpty) gives "a & b & c"
+    colonSeparated(sourcesABCAndEmpty) gives "a: b: c"
+    semicolonSeparated(sourcesABCAndEmpty) gives "a; b; c"
+    newLineSeparated(sourcesABCAndEmpty) gives "a\nb\nc"
+    emptyLineSeparated(sourcesABCAndEmpty) gives "a\n\nb\n\nc"
+  }
+
+  @Test
+  fun sourceInCurlyBrackets() {
+    inCurlyBrackets(source("")) gives "{}"
+    inCurlyBrackets(source("\n")) gives "{\n \n}"
+    inCurlyBrackets(source("\nfoo")) gives "{\n foo\n}"
+    inCurlyBrackets(source("\nfoo\nbar")) gives "{\n foo\n bar\n}"
+    inCurlyBrackets(source("foo\nbar")) gives "{foo\n bar\n}"
+  }
+
+  @Test
+  fun bracketedSources() {
+    inParentheses(sourceA) gives "(a)"
+    inAngleBrackets(sourceA) gives "<a>"
+    inSquareBrackets(sourceA) gives "[a]"
+    inDoubleQuotes(sourceA) gives "\"a\""
+    inInlineCurlyBrackets(sourceA) gives "{ a }"
+
+    inParenthesesIfNotEmpty(emptySource) gives ""
+    inParenthesesIfNotEmpty(sourceA) gives "(a)"
+  }
+
+  @Test
+  fun blockSource() {
+    block(emptySource) gives "{}"
+    block(sourceA) gives "{\n a\n}"
+    block(newLineSeparated(sourceA, sourceB)) gives "{\n a\n b\n}"
+  }
+
+  @Test
+  fun infixSource() {
+    infix(sourceA, "op", sourceB) gives "a op b"
+  }
+
+  @Test
+  fun sourceIf() {
+    sourceIf(true) { sourceA } gives "a"
+    sourceIf(false) { sourceA } gives ""
+  }
+
+  @Test
+  fun sourceIfEmpty() {
+    sourceA.ifEmpty { sourceB } gives "a"
+    emptySource.ifEmpty { sourceB } gives "b"
+  }
+
+  @Test
+  fun sourceIfNotEmpty() {
+    sourceA.ifNotEmpty(::inParentheses) gives "(a)"
+    emptySource.ifNotEmpty(::inParentheses) gives ""
+  }
+
+  @Test
+  fun ifNotNullSource() {
+    128.ifNotNullSource { source(it.inc().toString()) } gives "129"
+    (null as Int?).ifNotNullSource { source(it.inc().toString()) } gives ""
+  }
+}