diff --git a/decimal.d.ts b/decimal.d.ts
index ddc1de6..19ca1a7 100644
--- a/decimal.d.ts
+++ b/decimal.d.ts
@@ -251,7 +251,7 @@ export declare class Decimal {
   static exp(n: Decimal.Value): Decimal;
   static floor(n: Decimal.Value): Decimal;
   static hypot(...n: Decimal.Value[]): Decimal;
-  static isDecimal(object: any): boolean
+  static isDecimal(object: any): object is Decimal;
   static ln(n: Decimal.Value): Decimal;
   static log(n: Decimal.Value, base?: Decimal.Value): Decimal;
   static log2(n: Decimal.Value): Decimal;
diff --git a/decimal.global.d.ts b/decimal.global.d.ts
index 8428911..8cdac8c 100644
--- a/decimal.global.d.ts
+++ b/decimal.global.d.ts
@@ -272,7 +272,7 @@ export declare class Decimal {
   static exp(n: DecimalValue): Decimal;
   static floor(n: DecimalValue): Decimal;
   static hypot(...n: DecimalValue[]): Decimal;
-  static isDecimal(object: any): boolean
+  static isDecimal(object: any): object is Decimal;
   static ln(n: DecimalValue): Decimal;
   static log(n: DecimalValue, base?: DecimalValue): Decimal;
   static log2(n: DecimalValue): Decimal;