At a press conference in Dublin, Bryan said he was quitting so he could spend more time with his family.
He is married to I'm A Celeb winner and former Atomic Kitten singer Kerry McFadden. They have two daughters.
Bryan said his decision had nothing to do with the other lads. "They are my four best friends in the world. We have done so much over the past six years."
"It's very hard to juggle two lives when you have got a family.
"For the last year, since Lily (his second daughter) was born, I haven't really been able to commit 100% to Westlife and 100% to my family.
"I feel it is the right time for me to concentrate on my family."
The remaining lads - Shane, Kian, Nicky and Mark - wished Bryan the best of luck and vowed to continue building on the band's success.
They are set to kick off a world tour of more than 70 dates on 29 March.
Many CBBC Newsround Online readers have vowed to keep on supporting the group.
"I will miss Bryan because he is the funniest person in the group. I really hope the rest of band will carry on, but it will not be the same without him," said Chloe from Aberdeenshire.
Carole from Leeds said: "This is a sad day for Westlife, but they will have my continued support."
Others were upset that they won't be seeing Bryan on stage.
Rosemary from Banff said: "I'm absolutely gutted that Bryan's left. I wish he would still do the tour as so many of my friends have bought tickets."