diff --git a/modules/kxs-ts-gen-core/src/commonMain/kotlin/dev/adamko/kxstsgen/KxsTsAnnotations.kt b/modules/kxs-ts-gen-core/src/commonMain/kotlin/dev/adamko/kxstsgen/KxsTsAnnotations.kt
new file mode 100644
index 00000000..28621dd2
--- /dev/null
+++ b/modules/kxs-ts-gen-core/src/commonMain/kotlin/dev/adamko/kxstsgen/KxsTsAnnotations.kt
@@ -0,0 +1,18 @@
+package dev.adamko.kxstsgen
+
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.InheritableSerialInfo
+
+/**
+ * Marks a property as required in the generated TypeScript interface.
+ *
+ * This annotation is inheritable, so it should be sufficient to place it on a base class of hierarchy.
+ *
+ * This annotation should only be used if [kotlinx.serialization.json.JsonConfiguration.encodeDefaults]
+ * is set to true. If it is false (which it is by default), then properties with default values are
+ * potentially omitted from the generated JSON.
+ */
+@InheritableSerialInfo
+@Target(AnnotationTarget.PROPERTY)
+@ExperimentalSerializationApi
+annotation class KxsTsRequired
diff --git a/modules/kxs-ts-gen-core/src/commonMain/kotlin/dev/adamko/kxstsgen/core/TsElementConverter.kt b/modules/kxs-ts-gen-core/src/commonMain/kotlin/dev/adamko/kxstsgen/core/TsElementConverter.kt
index 9d9c4b7b..2da10dc9 100644
--- a/modules/kxs-ts-gen-core/src/commonMain/kotlin/dev/adamko/kxstsgen/core/TsElementConverter.kt
+++ b/modules/kxs-ts-gen-core/src/commonMain/kotlin/dev/adamko/kxstsgen/core/TsElementConverter.kt
@@ -1,5 +1,6 @@
 package dev.adamko.kxstsgen.core
 
+import dev.adamko.kxstsgen.KxsTsRequired
 import kotlinx.serialization.descriptors.PolymorphicKind
 import kotlinx.serialization.descriptors.PrimitiveKind
 import kotlinx.serialization.descriptors.SerialDescriptor
@@ -207,7 +208,9 @@ fun interface TsElementConverter {
       return descriptor.elementDescriptors.mapIndexed { index, fieldDescriptor ->
         val name = descriptor.getElementName(index)
         val fieldTypeRef = typeRefConverter(fieldDescriptor)
-        val optional = descriptor.isElementOptional(index)
+        val optional =
+          descriptor.getElementAnnotations(index).none { it is KxsTsRequired }
+            && descriptor.isElementOptional(index)
         TsProperty(name, fieldTypeRef, optional)
       }.toSet()
     }
diff --git a/modules/kxs-ts-gen-core/src/commonTest/kotlin/dev/adamko/kxstsgen/core/KxsTsGeneratorTest.kt b/modules/kxs-ts-gen-core/src/commonTest/kotlin/dev/adamko/kxstsgen/core/KxsTsGeneratorTest.kt
new file mode 100644
index 00000000..f6b7bf91
--- /dev/null
+++ b/modules/kxs-ts-gen-core/src/commonTest/kotlin/dev/adamko/kxstsgen/core/KxsTsGeneratorTest.kt
@@ -0,0 +1,45 @@
+package dev.adamko.kxstsgen.core
+
+import dev.adamko.kxstsgen.KxsTsConfig
+import dev.adamko.kxstsgen.KxsTsGenerator
+import dev.adamko.kxstsgen.KxsTsRequired
+import io.kotest.core.spec.style.FunSpec
+import io.kotest.matchers.shouldBe
+import kotlinx.serialization.Serializable
+
+class KxsTsGeneratorTest : FunSpec({
+  val config = KxsTsConfig(indent = "  ")
+  val tsGenerator = KxsTsGenerator(config)
+
+  test("Can make default-valued properties required") {
+    @Serializable
+    data class OptionalTest(
+      val optional: Boolean = false,
+      @KxsTsRequired
+      val required: Boolean = true,
+    )
+
+    tsGenerator.generate(OptionalTest.serializer()) shouldBe """
+      |export interface OptionalTest {
+      |  optional?: boolean;
+      |  required: boolean;
+      |}
+    """.trimMargin()
+  }
+
+  test("Can make nullable properties required") {
+    @Serializable
+    data class OptionalTest(
+      @KxsTsRequired
+      val required: Boolean? = null,
+      val optional: Boolean? = null,
+    )
+
+    tsGenerator.generate(OptionalTest.serializer()) shouldBe """
+      |export interface OptionalTest {
+      |  required: boolean | null;
+      |  optional?: boolean | null;
+      |}
+    """.trimMargin()
+  }
+})