The Saturdays have said they are "thrilled" after scoring their first number one hit, which also became the fastest-selling single of the year.
The five-piece girl group had previously notched up 11 top 10 singles, three top 40 albums and 3.5 million record sales, but the closest they had ever got to the top spot before was number two.
What About Us, featuring Sean Paul, sold 114,000 copies - more than 40,000 copies ahead of Justin Timberlake, at number two with Mirrors, the Official Charts Company said.
The Saturdays told Officialcharts.com: "We couldn't be more thrilled to be the UK's official new number one with What About Us."
The rest of the top five stayed unchanged from last week, with Pink at number three with Just Give Me A Reason, Bruno Mars at number four with When I Was Your Man and Bastille's Pompeii at number five.
There were new entries for Kodaline at number 16 with High Hopes, while Feel This Moment by Pitbull featuring Christina Aguilera debuted at number 27, Reconnected's One In A Million entered at number 32 and Fade by Jakwob featuring Maiday at number 35.
Despite losing his number one in the singles chart, there was consolation for Timberlake as he stole the album chart top spot from David Bowie with The 20/20 Experience - the fastest-selling album of the year.
Bowie's album The Next Day, his first new material in nearly a decade, went down a place to number two.
Emile Sande's Our Version Of Events stayed at number three, with the soundtrack to Oscar-winning film Les Miserables rising up the charts from number 10 to number four. Bastille's Bad Blood fell one position to number five.
The new album from Suede, Bloodsports, entered the charts at number 10, while Billy Bragg's Tooth And Nail was a new entry at 13.