Delivering a tray of tea to reporters standing in the snow outside his north London home, Ross said: "It's probably not a bad time for me to move on - and it's probably not a bad time for the BBC, either.
"I've got six months left, I'm hoping to make the best shows of my career with them."
One of the biggest names at the BBC, Ross currently hosts his own Friday night chat show, Radio 2 show and a film review programme.
The announcement comes a day after it was revealed that Graham Norton had signed a two-year deal with the corporation prompting newspaper speculation that he would take over Ross's Friday night slot on BBC One.
But BBC creative director Alan Yentob said it was "too premature" to make such decisions.
He added: "You don't need to compare Graham with Jonathan Ross. No decisions have been taken with that slot."
It was reported that Ross's contract, which secured his services for the three years up to July 2010, was worth £18m.
The BBC has never confirmed that amount, but it was expected that Ross would be asked to accept a pay cut, following the lead of other big stars, such as Bruce Forsyth and Chris Moyles.
"Although I have had a wonderful time working for the BBC, and am very proud of the shows I have made while there, over the last two weeks I have decided not to re-negotiate when my current contract comes to an end," Ross said.
Torin Douglas, Media correspondent
This has come out of the blue. For Ross to give up his radio as well as his television shows is what has really surprised people, because he loved radio and the Radio 2 show was a big success.
Since "Sachsgate" he has reined back, and when he reins back you realise what a good broadcaster he is. He manages to attract a young audience, even though he's getting older himself, but still get a mainstream audience. That's why he's a very valuable property.
A lot of people hate him and think he stands for all the things that the BBC should not stand for, but lots of others love him. Channel 4 will be very interested in trying to get him.
Graham Norton could be a ready-made TV replacement - in that way the BBC is well placed. For the Film 2010 show, Mark Kermode from 5 live is a potential successor. But replacing Ross on radio will be harder.
"While there, I have worked with some of the nicest and most talented people in the industry and had the opportunity to interview some of the biggest stars in the world, and am grateful to the BBC for such a marvellous experience.
"I would like to make it perfectly clear that no negotiations ever took place and that my decision is not financially motivated.
He said that he had previously turned down "more lucrative offers from other channels" because the BBC was "where I wanted to be".
He added: "As I have said before - I would happily have stayed there for any fee they cared to offer, but there were other considerations.
"I love making my Friday night talk show, my Saturday morning radio show and the Film Programme, and will miss them all."
The star said he will remain working for the BBC until the summer and will continue hosting the Bafta Film Awards, Comic Relief and other BBC specials.
"Working at the BBC has been a tremendous privilege, and I would like to thank everyone who has watched and listened so loyally over the last 13 years."
BBC Creative Director Alan Yentob discusses why Jonathan Ross is leaving
Jana Bennett, director of BBC Vision, said she could "understand" Ross's decision "following a difficult year".
She called him an "extremely talented broadcaster" and said all his programmes had been a "great success".
"I'm pleased that Jonathan will continue to apply his considerable abilities to the remaining six months of Friday Night, Film 2010 and his Radio 2 show," she said.
"I'm delighted that he will continue to present the Bafta awards and Comic Relief for BBC Television."
Ross has thanked fans posting on micro-blogging site Twitter, "Thanks for all the kind words about my decision. I feel sad that I can't keep making the shows so many of you love!"
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