Branch employees will take part in the World's Biggest Coffee Morning to celebrate, and open their doors to customers to pop in for a cup of tea or coffee and home-baked organic cakes and cookies.
The team hope to raise £2,000 on the day of the event on September 26.
Alise Matule, customer representative at Nationwide said: "We are delighted to be supporting Macmillan by hosting World Biggest Coffee Morning on 26th September 2014, it's a fantastic cause and we are really excited about organic tea morning.
"My mother is directly affected by secondary metastatic liver cancer and from my personal experience it's amazing that charities like this exist to support the ones in need, especially as Nationwide is celebrating this anniversary with the charity this year."
Over the last 20 years Nationwide has raised more than £6.5 million for Macmillan, helping them to support thousands of people living with cancer through a range of fundraising activities.
This year marks the 23rd birthday of the world's biggest coffee morning - the charity's most successful fundraising event.
Lucy Groen, partnership account manager at Macmillan said:" I would like to thank every one at Nationwide for their ongoing support for Macmillan over the last two decades, and hope all the employees and customers have a great day of fundraising.
"The money you raise at your Words Biggest Coffee Morning will go towards helping Macmillan insure that no one faces cancer alone."
Nationwide will welcome people from 11am on September 26 at 156 The Centre, Feltham.
For more information call the Feltham Nationwide team on 0845 266 0235 or email email@example.com
To be a part of the day visit www.macmillan.org.uk/coffee or call 0845 602 1246 or text COFFEE to 88080 to get your Coffee Morning Kit today.