diff --git a/.changelog/21363.txt b/.changelog/21363.txt
new file mode 100644
index 00000000000..7339a4bc6e8
--- /dev/null
+++ b/.changelog/21363.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+resource/aws_cloudwatch_metric_alarm: Fix imported 'treat_missing_data' diff
+```
diff --git a/internal/service/cloudwatch/enum.go b/internal/service/cloudwatch/enum.go
new file mode 100644
index 00000000000..f017be18f25
--- /dev/null
+++ b/internal/service/cloudwatch/enum.go
@@ -0,0 +1,29 @@
+package cloudwatch
+
+const (
+	lowSampleCountPercentilesEvaluate          = "evaluate"
+	lowSampleCountPercentilesmissingDataIgnore = "ignore"
+)
+
+func lowSampleCountPercentiles_Values() []string {
+	return []string{
+		lowSampleCountPercentilesEvaluate,
+		lowSampleCountPercentilesmissingDataIgnore,
+	}
+}
+
+const (
+	missingDataBreaching    = "breaching"
+	missingDataIgnore       = "ignore"
+	missingDataMissing      = "missing"
+	missingDataNotBreaching = "notBreaching"
+)
+
+func missingData_Values() []string {
+	return []string{
+		missingDataBreaching,
+		missingDataIgnore,
+		missingDataMissing,
+		missingDataNotBreaching,
+	}
+}
diff --git a/internal/service/cloudwatch/metric_alarm.go b/internal/service/cloudwatch/metric_alarm.go
index db31375f6d3..f3fdd443bee 100644
--- a/internal/service/cloudwatch/metric_alarm.go
+++ b/internal/service/cloudwatch/metric_alarm.go
@@ -240,14 +240,14 @@ func ResourceMetricAlarm() *schema.Resource {
 			"treat_missing_data": {
 				Type:         schema.TypeString,
 				Optional:     true,
-				Default:      "missing",
-				ValidateFunc: validation.StringInSlice([]string{"breaching", "notBreaching", "ignore", "missing"}, true),
+				Default:      missingDataMissing,
+				ValidateFunc: validation.StringInSlice(missingData_Values(), true),
 			},
 			"evaluate_low_sample_count_percentiles": {
 				Type:         schema.TypeString,
 				Optional:     true,
 				Computed:     true,
-				ValidateFunc: validation.StringInSlice([]string{"evaluate", "ignore"}, true),
+				ValidateFunc: validation.StringInSlice(lowSampleCountPercentiles_Values(), true),
 			},
 
 			"tags":     tftags.TagsSchema(),
@@ -357,7 +357,11 @@ func resourceMetricAlarmRead(d *schema.ResourceData, meta interface{}) error {
 	d.Set("threshold_metric_id", resp.ThresholdMetricId)
 	d.Set("unit", resp.Unit)
 	d.Set("extended_statistic", resp.ExtendedStatistic)
-	d.Set("treat_missing_data", resp.TreatMissingData)
+	if resp.TreatMissingData != nil {
+		d.Set("treat_missing_data", resp.TreatMissingData)
+	} else {
+		d.Set("treat_missing_data", missingDataMissing)
+	}
 	d.Set("evaluate_low_sample_count_percentiles", resp.EvaluateLowSampleCountPercentile)
 
 	tags, err := ListTags(conn, arn)
diff --git a/internal/service/cloudwatch/metric_alarm_test.go b/internal/service/cloudwatch/metric_alarm_test.go
old mode 100644
new mode 100755
index acbdf1afa2e..b5894668a4b
--- a/internal/service/cloudwatch/metric_alarm_test.go
+++ b/internal/service/cloudwatch/metric_alarm_test.go
@@ -205,6 +205,18 @@ func TestAccCloudWatchMetricAlarm_treatMissingData(t *testing.T) {
 					resource.TestCheckResourceAttr(resourceName, "treat_missing_data", "breaching"),
 				),
 			},
+			{
+				Config: testAccMetricAlarmTreatMissingDataConfigNoAttr(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckCloudWatchMetricAlarmExists(resourceName, &alarm),
+					resource.TestCheckResourceAttr(resourceName, "treat_missing_data", "missing"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
 		},
 	})
 }
@@ -537,6 +549,27 @@ resource "aws_cloudwatch_metric_alarm" "test" {
 `, rName)
 }
 
+func testAccMetricAlarmTreatMissingDataConfigNoAttr(rName string) string {
+	return fmt.Sprintf(`
+resource "aws_cloudwatch_metric_alarm" "test" {
+  alarm_name                = "%s"
+  comparison_operator       = "GreaterThanOrEqualToThreshold"
+  evaluation_periods        = "2"
+  metric_name               = "CPUUtilization"
+  namespace                 = "AWS/EC2"
+  period                    = "120"
+  statistic                 = "Average"
+  threshold                 = "80"
+  alarm_description         = "This metric monitors ec2 cpu utilization"
+  insufficient_data_actions = []
+
+  dimensions = {
+    InstanceId = "i-abc123"
+  }
+}
+`, rName)
+}
+
 func testAccMetricAlarmTreatEvaluateLowSampleCountPercentilesConfig(rName string) string {
 	return fmt.Sprintf(`
 resource "aws_cloudwatch_metric_alarm" "test" {