The app, which can be downloaded free from the App Store or Watch App Store, offers access to live daily exchange rates for up to 70 currencies.
Users can activate it by voice – saying “how much is that” – or by tapping to find out how much an item costs when they are overseas.
It can also be used by overseas visitors allowing them to, calculate their costs whilst visiting the UK.
When abroad, the app can still be used offline as rates and currency values are stored.
“Our new free currency converter app is a great tool for holidaymakers this summer, helping people budget before, during and after their holiday,” said The Post Office.
“The simple currency converter allows the Apple Watch wearer to find out how much something costs instantly, whether that’s a cup of coffee in Paris or an ice cream in Majorca.”
Post Office buy-back exchange rates have also been added to the Apple Watch App, allowing tourists to calculate how much their unused currency is worth.