diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3ec93da..3dfec4c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - `isValidSwissSocialInsuranceNumber` is now named properly
+- `isValidSwissIbanNumber` now also allows IBAN numbers with letters
 
 ## [2.1.0] - 2025-09-03
 
diff --git a/src/lib/swissStandards.spec.ts b/src/lib/swissStandards.spec.ts
index 30f3df6..ec29423 100644
--- a/src/lib/swissStandards.spec.ts
+++ b/src/lib/swissStandards.spec.ts
@@ -11,8 +11,21 @@ describe("Swiss standards test", () => {
     ["CH93 0000 0000 0000 0000 1", false],
     ["ch93 0076 2011 6238 5295 7", false],
     ["DE93 0076 2011 6238 5295 7", false],
-  ])("check if this swiss IBAN is valid or not", (unformattedIbanNumber, expected) => {
-    expect(isValidSwissIbanNumber(unformattedIbanNumber)).toBe(expected);
+  ])("check if the swiss IBAN number is valid or not", (iBanNumberToCheck, expected) => {
+    expect(isValidSwissIbanNumber(iBanNumberToCheck)).toBe(expected);
+  });
+
+  test.each([
+    [null as unknown as string, false],
+    [undefined as unknown as string, false],
+    ["CH3400762ABC123DEF456", true],
+    ["CH34 0076 2ABC 123D EF45 6", true],
+    ["Some random string", false],
+    ["DE34 0076 2ABC 123D EF45 3", false],
+    ["CH34 0076 2ABC 123D EF45 \n6", false],
+    ["CH34 0076 2ABC 123D EF45 !", false],
+  ])("check if the siwss IBAN number with letters is valid or not", (iBanNumberToCheck, expected) => {
+    expect(isValidSwissIbanNumber(iBanNumberToCheck)).toBe(expected);
   });
 
   test.each([
@@ -26,7 +39,7 @@ describe("Swiss standards test", () => {
     ["756.1234.5678.91", false],
     ["test756.9217.0769.85", false],
     ["7.56..9217...0769.85", false],
-  ])("check if the social insurance number is valid or not", (ahvNumber, expected) => {
-    expect(isValidSwissSocialInsuranceNumber(ahvNumber)).toBe(expected);
+  ])("check if the social insurance number is valid or not", (socialInsuranceNumberToCheck, expected) => {
+    expect(isValidSwissSocialInsuranceNumber(socialInsuranceNumberToCheck)).toBe(expected);
   });
 });
diff --git a/src/lib/swissStandards.ts b/src/lib/swissStandards.ts
index cfcaa68..3fd7d13 100644
--- a/src/lib/swissStandards.ts
+++ b/src/lib/swissStandards.ts
@@ -16,12 +16,12 @@ export function isValidSwissIbanNumber(ibanNumber: string): boolean {
 
   // 2. Define allowed strict formats
   //    - with spaces: "CHXX XXXX XXXX XXXX XXXX X"
-  const compactIbanNumberWithWhiteSpaces = new RegExp(/^CH\d{2}(?: \d{4}){4} \d{1}$/);
+  const compactIbanNumberWithWhiteSpaces = new RegExp(/^CH[0-9]{2} [0-9]{4} [0-9][A-Z0-9]{3} [A-Z0-9]{4} [A-Z0-9]{4} [A-Z0-9]$/);
   //    - without spaces: "CHXXXXXXXXXXXXXXXXXXX"
-  const compactIbanNumberWithoutWhiteSpaces = new RegExp(/^CH\d{19}$/);
+  const compactIbanNumberWithoutWhiteSpaces = new RegExp(/^CH[0-9]{7}[A-Z0-9]{12}$/);
 
-  // 3. Check if input matches one of the allowed formats
-  if (!compactIbanNumberWithWhiteSpaces.test(ibanNumber) && !compactIbanNumberWithoutWhiteSpaces.test(ibanNumber)) {
+  // 3. Check if the input matches one of the allowed formats
+  if (!(compactIbanNumberWithWhiteSpaces.test(ibanNumber) || compactIbanNumberWithoutWhiteSpaces.test(ibanNumber))) {
     return false;
   }
 
@@ -46,7 +46,7 @@ export function isValidSwissIbanNumber(ibanNumber: string): boolean {
 }
 
 /**
- * Validation of social insurance number with checking the checksum
+ * Validation of a social insurance number with checking the checksum
  * Validation according to https://www.sozialversicherungsnummer.ch/aufbau-neu.htm
  * @param socialInsuranceNumber The social insurance number to check
  * Must be in one of the following formats:
@@ -55,13 +55,13 @@ export function isValidSwissIbanNumber(ibanNumber: string): boolean {
  * @returns The result if the social insurance number is valid or not
  */
 export function isValidSwissSocialInsuranceNumber(socialInsuranceNumber: string): boolean {
-  // 1. Check if input is empty or only whitespace
+  // 1. Check if the input is empty or only a whitespace
   if (isNullOrWhitespace(socialInsuranceNumber)) {
     return false;
   }
 
   /**
-   * 2. Check if input matches accepted formats:
+   * 2. Check if the input matches one of the accepted formats:
    *    - With dots: 756.XXXX.XXXX.XX
    *    - Without dots: 756XXXXXXXXXX
    */