diff --git a/wire-protoc-compatibility-tests/src/test/java/com/squareup/wire/Proto3WireProtocCompatibilityTests.kt b/wire-protoc-compatibility-tests/src/test/java/com/squareup/wire/Proto3WireProtocCompatibilityTests.kt
index a5f43f007a..e863fcede7 100644
--- a/wire-protoc-compatibility-tests/src/test/java/com/squareup/wire/Proto3WireProtocCompatibilityTests.kt
+++ b/wire-protoc-compatibility-tests/src/test/java/com/squareup/wire/Proto3WireProtocCompatibilityTests.kt
@@ -518,14 +518,14 @@ class Proto3WireProtocCompatibilityTests {
     val googleMessage = PizzaOuterClass.PizzaDelivery.newBuilder()
       .setOrderedAt(
         Timestamp.newBuilder()
-          .setSeconds(-631152000000L) // 1950-01-01T00:00:00.250Z.
+          .setSeconds(-631152000L) // 1950-01-01T00:00:00.250Z.
           .setNanos(250_000_000)
           .build(),
       )
       .build()
 
     val wireMessage = PizzaDeliveryK(
-      ordered_at = ofEpochSecond(-631152000000L, 250_000_000L),
+      ordered_at = ofEpochSecond(-631152000L, 250_000_000L),
     )
 
     val googleMessageBytes = googleMessage.toByteArray()
diff --git a/wire-runtime/src/commonMain/kotlin/com/squareup/wire/Instant.kt b/wire-runtime/src/commonMain/kotlin/com/squareup/wire/Instant.kt
index 8bdb1be5ed..b3cce0439d 100644
--- a/wire-runtime/src/commonMain/kotlin/com/squareup/wire/Instant.kt
+++ b/wire-runtime/src/commonMain/kotlin/com/squareup/wire/Instant.kt
@@ -27,6 +27,9 @@ expect class Instant {
    *
    * For example, this value will be -1 for the instant 1969-12-31T23:59:59Z, and 1 for the instant
    * 1970-01-01T00:00:01Z.
+   *
+   * It must be between -62135596800 and 253402300799 inclusive (which corresponds to
+   * 0001-01-01T00:00:00Z to 9999-12-31T23:59:59Z).
    */
   fun getEpochSecond(): Long
 
diff --git a/wire-runtime/src/commonMain/kotlin/com/squareup/wire/ProtoAdapter.kt b/wire-runtime/src/commonMain/kotlin/com/squareup/wire/ProtoAdapter.kt
index 26553d44ba..38257194f5 100644
--- a/wire-runtime/src/commonMain/kotlin/com/squareup/wire/ProtoAdapter.kt
+++ b/wire-runtime/src/commonMain/kotlin/com/squareup/wire/ProtoAdapter.kt
@@ -1277,6 +1277,20 @@ internal fun commonDuration(): ProtoAdapter<Duration> = object : ProtoAdapter<Du
     }
 }
 
+// Protobuf Timestamp valid range: 0001-01-01T00:00:00Z to 9999-12-31T23:59:59.999999999Z inclusive.
+private const val TIMESTAMP_SECONDS_MIN = -62135596800L
+private const val TIMESTAMP_SECONDS_MAX = 253402300799L
+
+@Suppress("NOTHING_TO_INLINE")
+private inline fun checkTimestampRange(seconds: Long, nanos: Int) {
+  require(seconds in TIMESTAMP_SECONDS_MIN..TIMESTAMP_SECONDS_MAX) {
+    "Timestamp seconds ($seconds) must be in range [$TIMESTAMP_SECONDS_MIN, $TIMESTAMP_SECONDS_MAX]"
+  }
+  require(nanos in 0..999_999_999) {
+    "Timestamp nanos ($nanos) must be in range [0, 999999999]"
+  }
+}
+
 internal fun commonInstant(): ProtoAdapter<Instant> = object : ProtoAdapter<Instant>(
   LENGTH_DELIMITED,
   Instant::class,
@@ -1284,25 +1298,28 @@ internal fun commonInstant(): ProtoAdapter<Instant> = object : ProtoAdapter<Inst
   Syntax.PROTO_3,
 ) {
   override fun encodedSize(value: Instant): Int {
-    var result = 0
     val seconds = value.getEpochSecond()
-    if (seconds != 0L) result += INT64.encodedSizeWithTag(1, seconds)
     val nanos = value.getNano()
+    checkTimestampRange(seconds, nanos)
+    var result = 0
+    if (seconds != 0L) result += INT64.encodedSizeWithTag(1, seconds)
     if (nanos != 0) result += INT32.encodedSizeWithTag(2, nanos)
     return result
   }
 
   override fun encode(writer: ProtoWriter, value: Instant) {
     val seconds = value.getEpochSecond()
-    if (seconds != 0L) INT64.encodeWithTag(writer, 1, seconds)
     val nanos = value.getNano()
+    checkTimestampRange(seconds, nanos)
+    if (seconds != 0L) INT64.encodeWithTag(writer, 1, seconds)
     if (nanos != 0) INT32.encodeWithTag(writer, 2, nanos)
   }
 
   override fun encode(writer: ReverseProtoWriter, value: Instant) {
+    val seconds = value.getEpochSecond()
     val nanos = value.getNano()
+    checkTimestampRange(seconds, nanos)
     if (nanos != 0) INT32.encodeWithTag(writer, 2, nanos)
-    val seconds = value.getEpochSecond()
     if (seconds != 0L) INT64.encodeWithTag(writer, 1, seconds)
   }
 
diff --git a/wire-runtime/src/commonTest/kotlin/com/squareup/wire/ProtoAdapterTest.kt b/wire-runtime/src/commonTest/kotlin/com/squareup/wire/ProtoAdapterTest.kt
index 7567959fc7..9651097d8d 100644
--- a/wire-runtime/src/commonTest/kotlin/com/squareup/wire/ProtoAdapterTest.kt
+++ b/wire-runtime/src/commonTest/kotlin/com/squareup/wire/ProtoAdapterTest.kt
@@ -16,6 +16,8 @@
 package com.squareup.wire
 
 import assertk.assertFailure
+import assertk.assertThat
+import assertk.assertions.isEqualTo
 import assertk.assertions.isInstanceOf
 import kotlin.test.Test
 
@@ -44,4 +46,44 @@ class ProtoAdapterTest {
       ProtoAdapter.BOOL.asPacked().asRepeated()
     }.isInstanceOf<UnsupportedOperationException>()
   }
+
+  @Test fun instantEncodeValidMinBoundary() {
+    // 0001-01-01T00:00:00Z.
+    val instant = ofEpochSecond(-62135596800L, 0L)
+    val bytes = ProtoAdapter.INSTANT.encode(instant)
+    assertThat(ProtoAdapter.INSTANT.decode(bytes).getEpochSecond()).isEqualTo(-62135596800L)
+  }
+
+  @Test fun instantEncodeValidMaxBoundary() {
+    // 9999-12-31T23:59:59Z.
+    val instant = ofEpochSecond(253402300799L, 999_999_999L)
+    val bytes = ProtoAdapter.INSTANT.encode(instant)
+    val decoded = ProtoAdapter.INSTANT.decode(bytes)
+    assertThat(decoded.getEpochSecond()).isEqualTo(253402300799L)
+    assertThat(decoded.getNano()).isEqualTo(999_999_999)
+  }
+
+  @Test fun instantEncodeRejectsSecondsBelowMin() {
+    // 0001-01-01T00:00:00Z - 1 second.
+    val instant = ofEpochSecond(-62135596801L, 0L)
+    assertFailure {
+      ProtoAdapter.INSTANT.encode(instant)
+    }.isInstanceOf<IllegalArgumentException>()
+  }
+
+  @Test fun instantEncodeRejectsSecondsAboveMax() {
+    // 9999-12-31T23:59:59Z + 1 second.
+    val instant = ofEpochSecond(253402300800L, 0L)
+    assertFailure {
+      ProtoAdapter.INSTANT.encode(instant)
+    }.isInstanceOf<IllegalArgumentException>()
+  }
+
+  @Test fun instantEncodedSizeRejectsOutOfRange() {
+    // 0001-01-01T00:00:00Z - 1 second.
+    val instant = ofEpochSecond(-62135596801L, 0L)
+    assertFailure {
+      ProtoAdapter.INSTANT.encodedSize(instant)
+    }.isInstanceOf<IllegalArgumentException>()
+  }
 }