The 26-year-old One Direction star was confirmed yesterday and will join singing legend Robbie Williams as one of the new faces on the judging panel when the hit ITV show returns this summer.
The panel will be completed by Williams' wife Ayda Field and long-term judge Simon Cowell.
The reshuffle comes after long-standing judge Louis Walsh quit the show last month after 13 years.
Sharon Osbourne will also be a judge - but only on the live shows that follow the pre-filmed auditions.
The new panel was unveiled in London on Tuesday ahead of the auditions, which are set to begin later this week.
Louis Tomlinson found fame on the show in 2010 with his band One Direction becoming a global success on the back of the programme.
He has previously appeared as a guest judge at judges' houses in 2015, helping Simon Cowell select which of his acts he would take through to the live shows.
The new foursome have been unveiled as part of a series of changes introduced ahead of the upcoming 15th season, made in a bid to reverse falling ratings.
Judges' auditions for the new series will take place at Wembley Arena and more than 30,000 contestants have already taken part in open auditions held across the country.