The bank will open 100 branches between 11am and 3pm every Sunday. A total of 200 branches will also be open for the full day on Saturday, between 9 am and 4pm.
Dick Spelman, director of distribution, said: "Full day opening at over 200 of our branches on Saturday is allowing us to take full advantage of the busiest shopping day in the week.
"Sunday opening in the 100 selected branches allows us to provide a high street service for the significant number of our customers who find it more convenient to do business with us on that day."
The move runs directly against the trend among other high street banks, where directors believe there is too little business on a Sunday to justify the cost of running a branch.
Between 1994 and 1996, NatWest opened every day in the run-up to Christmas. "We found they weren't used very much," a spokeswoman said. "People wanted cash but they didn't want to discuss loans, insurance or mortgages on a Sunday."