diff --git a/stackit/internal/services/observability/alertgroup/resource.go b/stackit/internal/services/observability/alertgroup/resource.go
index c4199cd7d..db2fe31fd 100644
--- a/stackit/internal/services/observability/alertgroup/resource.go
+++ b/stackit/internal/services/observability/alertgroup/resource.go
@@ -261,6 +261,7 @@ func (a *alertGroupResource) Schema(_ context.Context, _ resource.SchemaRequest,
 								mapvalidator.KeysAre(stringvalidator.LengthAtMost(200)),
 								mapvalidator.ValueStringsAre(stringvalidator.LengthAtMost(200)),
 								mapvalidator.SizeAtMost(5),
+								mapvalidator.ValueStringsAre(validate.NoLeadingOrTrailingWhitespace()),
 							},
 						},
 						"record": schema.StringAttribute{
diff --git a/stackit/internal/validate/validate.go b/stackit/internal/validate/validate.go
index 502187733..358fc6d3f 100644
--- a/stackit/internal/validate/validate.go
+++ b/stackit/internal/validate/validate.go
@@ -9,6 +9,7 @@ import (
 	"strings"
 	"time"
 	_ "time/tzdata"
+	"unicode"
 
 	"github.com/google/uuid"
 	"github.com/hashicorp/terraform-plugin-framework-validators/helpers/validatordiag"
@@ -394,3 +395,29 @@ func IsLowercased() *Validator {
 		},
 	}
 }
+
+// NoLeadingOrTrailingWhitespace returns a Validator that checks if the input string has leading or trailing whitespace.
+// Examples:
+// - "example": valid
+// - "": valid, empty value
+// - " example": invalid, leading whitespace
+// - "example ": invalid, trailing whitespace
+func NoLeadingOrTrailingWhitespace() *Validator {
+	description := "Value must not have leading or trailing whitespace, as defined by gos unicode.IsSpace(rune)."
+	return &Validator{
+		description: description,
+		validate: func(_ context.Context, request validator.StringRequest, response *validator.StringResponse) {
+			val := request.ConfigValue.ValueString()
+			if val == "" {
+				return
+			}
+			if unicode.IsSpace(rune(val[0])) || unicode.IsSpace(rune(val[len(val)-1])) {
+				response.Diagnostics.Append(validatordiag.InvalidAttributeValueDiagnostic(
+					request.Path,
+					description,
+					val,
+				))
+			}
+		},
+	}
+}
diff --git a/stackit/internal/validate/validate_test.go b/stackit/internal/validate/validate_test.go
index c08a70d14..60dd83b45 100644
--- a/stackit/internal/validate/validate_test.go
+++ b/stackit/internal/validate/validate_test.go
@@ -1008,3 +1008,60 @@ func TestIsLowercased(t *testing.T) {
 		})
 	}
 }
+
+func TestNoLeadingOrtTrailingWhitespace(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name    string
+		input   string
+		wantErr bool
+	}{
+		{
+			name:  "empty",
+			input: "",
+		},
+		{
+			name:  "valid",
+			input: "valid",
+		},
+		{
+			name:  "single char, valid",
+			input: "a",
+		},
+		{
+			name:    "leading whitespace",
+			input:   " leading",
+			wantErr: true,
+		},
+		{
+			name:    "trailing whitespace",
+			input:   "trailing ",
+			wantErr: true,
+		},
+		{
+			name:    "leading and trailing whitespace",
+			input:   " leading and trailing ",
+			wantErr: true,
+		},
+		{
+			name:    "single space",
+			input:   " ",
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			r := validator.StringResponse{}
+			NoLeadingOrTrailingWhitespace().ValidateString(context.Background(), validator.StringRequest{
+				ConfigValue: types.StringValue(tt.input),
+			}, &r)
+			if tt.wantErr && !r.Diagnostics.HasError() {
+				t.Fatalf("Expected validation to fail for input: %q", tt.input)
+			}
+			if !tt.wantErr && r.Diagnostics.HasError() {
+				t.Fatalf("Expected validation to succeed for input: %q, but got errors: %v", tt.input, r.Diagnostics.Errors())
+			}
+		})
+	}
+}