diff --git a/.changelog/4400.txt b/.changelog/4400.txt
new file mode 100644
index 0000000000..96dbd50cc6
--- /dev/null
+++ b/.changelog/4400.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+resource/mongodbatlas_advanced_cluster: Enables drift detection in `mongo_db_major_version` after performing an upgrade outside of Terraform
+```
diff --git a/internal/service/advancedcluster/resource_compatibility_test.go b/internal/service/advancedcluster/resource_compatibility_test.go
new file mode 100644
index 0000000000..3a914f83b3
--- /dev/null
+++ b/internal/service/advancedcluster/resource_compatibility_test.go
@@ -0,0 +1,42 @@
+package advancedcluster_test
+
+import (
+	"testing"
+
+	"github.com/mongodb/terraform-provider-mongodbatlas/internal/service/advancedcluster"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAdvancedCluster_ShouldUsePreviousMongoDBMajorVersion(t *testing.T) {
+	testCases := []struct {
+		name     string
+		before   string
+		after    string
+		expected bool
+	}{
+		{
+			name:     "keeps previous version when only formatting differs",
+			before:   "8",
+			after:    "8.0",
+			expected: true,
+		},
+		{
+			name:     "uses API value when major version actually changed",
+			before:   "7.0",
+			after:    "8.0",
+			expected: false,
+		},
+		{
+			name:     "keeps previous version when minor version differs within same major",
+			before:   "8.0",
+			after:    "8.2",
+			expected: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.expected, advancedcluster.ShouldUsePreviousMongoDBMajorVersion(tc.before, tc.after))
+		})
+	}
+}
diff --git a/internal/service/advancedcluster/resource_compatiblity.go b/internal/service/advancedcluster/resource_compatiblity.go
index e12156dd41..b023cf06ef 100644
--- a/internal/service/advancedcluster/resource_compatiblity.go
+++ b/internal/service/advancedcluster/resource_compatiblity.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"strconv"
+	"strings"
 
 	"go.mongodb.org/atlas-sdk/v20250312018/admin"
 
@@ -94,7 +95,9 @@ func overrideAttributesWithPrevStateValue(modelIn, modelOut *TFModel) {
 		return
 	}
 	beforeVersion := conversion.NilForUnknown(modelIn.MongoDBMajorVersion, modelIn.MongoDBMajorVersion.ValueStringPointer())
-	if beforeVersion != nil && !modelIn.MongoDBMajorVersion.Equal(modelOut.MongoDBMajorVersion) {
+	afterVersion := conversion.NilForUnknown(modelOut.MongoDBMajorVersion, modelOut.MongoDBMajorVersion.ValueStringPointer())
+	// Only preserve prior state value for formatting differences (e.g. "8" vs "8.0"), not real version transitions.
+	if beforeVersion != nil && afterVersion != nil && ShouldUsePreviousMongoDBMajorVersion(*beforeVersion, *afterVersion) {
 		modelOut.MongoDBMajorVersion = types.StringPointerValue(beforeVersion)
 	}
 	overrideMapStringWithPrevStateValue(&modelIn.Labels, &modelOut.Labels)
@@ -109,6 +112,18 @@ func overrideAttributesWithPrevStateValue(modelIn, modelOut *TFModel) {
 	modelOut.UseEffectiveFields = modelIn.UseEffectiveFields
 }
 
+func ShouldUsePreviousMongoDBMajorVersion(beforeVersion, afterVersion string) bool {
+	if beforeVersion == afterVersion {
+		return false
+	}
+	return majorComponent(beforeVersion) == majorComponent(afterVersion)
+}
+
+func majorComponent(version string) string {
+	major, _, _ := strings.Cut(FormatMongoDBMajorVersion(version), ".")
+	return major
+}
+
 func overrideMapStringWithPrevStateValue(mapIn, mapOut *types.Map) {
 	if mapIn == nil || mapOut == nil || len(mapOut.Elements()) > 0 {
 		return