Robin has been in a relationship with Anne since her split from Harry's father, Des, when the singer was seven. Robin and Anne live in Cheshire with Harry's sister Gemma, 20.
According to The Mirror, Harry is "thrilled" that Anne, 43, asked him to play such an important part in her wedding, and it's thought she has also invited her son's bandmates, Niall Horan, Louis Tomlinson, Liam Payne and Zayn Malik to her big day.
Robin's son Mike, from a previous relationship, will also take the role of best man on the day.
The wedding is believed to be taking place near the family's home in Holmes Chapel, although the exact location and time will only be announced to guests at the last minutes to help keep the ceremony private.
Harry, 19, is very close to his mum and often takes her to events as his guest.
"I tell her everything," he said in an interview. "She knows the truth."
Harry decided to give his mum some VIP treatment in February when he took her along as his plus one to the Brit Awards Sony and Belvedere afterparty at London's Arts Club.
Harry is not the only member of One Direction to take on the role of 'best man'. Niall Horan, 19, put his band commitments on hold in March to stand by his brother Greg's side at the altar.
The teen pin-up chose a dapper in a three-piece suit for his brother's big day at St Michael's Church in Castletown Geoghegan in County Westmeath, Ireland.
1D fans, who will no doubt flock to spot Harry on Saturday, showed up to wish the newlyweds well and catch a glimpse of the Irish singer. As he pulled up in his Range Rover with the groom, Niall made sure he greeted many of them in person, handing out hugs and autographs despite the cold.
One Direction are currently taking a well-deserved break after touring sixteen countries since February, where they played to hordes of fans in sold-out venues.