Niall Horan, Liam Payne, Harry Styles and Louis Tomlinson will continue as a four-piece, with plans for the band to continue on their world tour and to record their fifth album.
Zayn's exit come a week after he pulled out of the tour after being signed off with stress.
In a statement, Zayn said: "My life with One Direction has been more than I could ever have imagined. But, after five years, I feel like it is now the right time for me to leave the band. I'd like to apologise to the fans if I've let anyone down, but I have to do what feels right in my heart.
"I am leaving because I want to be a normal 22-year-old who is able to relax and have some private time out of the spotlight. I know I have four friends for life in Louis, Liam, Harry and Niall. I know they will continue to be the best band in the world."
Zayn's bandmates said they are "sad" to see him go, but "respect" his decision to leave.
The band said: "We're really sad to see Zayn go, but we totally respect his decision and send him all our love for the future. The past five years have been beyond amazing, we've gone through so much together, so we will always be friends.
"The four of us will now continue," they added. "We're looking forward to recording the new album and seeing all the fans on the next stage of the world tour."
One Direction's boss Simon Cowell, added: "I would like to say thank you to Zayn for everything he has done for One Direction. Since I first met Zayn in 2010, I have grown very, very fond - and immensely proud - of him.
"I have seen him grow in confidence and I am truly sorry to see him leave," Simon continued. "As for One Direction, fans can rest assured that Niall, Liam, Harry and Louis are hugely excited about the future of the band."
Zayn has had a difficult past week after pictures emerged of him holding hands with a blonde female who wasn't his Little Mix fiancee Perrie Edwards.
He broke his silence on Twitter, later saying: "I'm 22 years old... I love a girl named Perrie Edwards. And there's a lot of jealous f**ks in this world I'm sorry for what it looks like."
Watch the public - and some celebs! - react to the news:
Just days later, Zayn was signed off with stress, with a statement from the band's management saying: "Zayn has been signed off with stress and is flying back to the UK to recuperate. The band wish him well and will continue with their performances in Manila and Jakarta."
One Direction rose to fame on the seventh series of The X Factor in 2010. They all applied as solo contestants, but failed to qualify for the Boys category. Simon and Nicole Scherzinger then put them together to form a band.
Competing in the Groups category, One Direction finished in third place. They went on to achieve global success, making them one of the biggest boybands in the world.
For exclusive competitions and amazing prizes visit Reveal's Facebook page NOW!