The pledge will cover cash withdrawals made up until March 31 2022.
The move will help to provide vital services such as food bank provisions and financial guidance to help families reduce the impact of increasing household bills and unexpected costs this winter, the Post Office said.
With more than 1.5 million over-the-counter cash withdrawals every week, the Post Office hopes to raise £250,000.
The Trussell Trust supports a nationwide network of over 1,300 food bank centres.
Nick Read Post Office chief executive, said: “This can be an incredibly difficult time of year for many people who are struggling to make ends meet and facing impossible financial decisions.”
Emma Revie, chief executive of the Trussell Trust said: “This December, food banks in the Trussell Trust network face providing more than 7,000 food parcels every day, and we know that the vast majority of this support will be provided for people who are living in destitution – meaning they are unable to afford the essentials to eat, stay warm, dry and clean.”