Walker said in a Twitter video that he would also present "a whole host of new programmes across the channel".
The presenter will replace Sian Williams as the Channel 5 news anchor, who recently announced she was leaving.
Williams also made the move across from the BBC Breakfast sofa to Channel 5 in 2016. Walker began working on BBC Breakfast the same year.
He replaced Bill Turnbull and shared the sofa with Louise Minchin until her departure in September 2021. She was replaced by Sally Nugent.
Walker also previously presented the BBC's Saturday morning sports show Football Focus but handed the baton to Alex Scott last year.
In a video shared on Twitter, Walker said it was a "massive decision" to leave the BBC but said: "I don't think opportunities like this come around very often."
Walker added: "I'm really excited but this has also been a massive decision for me because I love BBC Breakfast, I love working alongside Sally and I love the team there.
"But Channel 5 came with big ambitions, with big plans and I don't think opportunities like this come around very often and I can't wait.
"And what an honour to step into the shoes of Sian Williams. I know how popular she is with both the team and also the audience, so all I can say is I will do my best to maintain her incredibly high standards.
"Thank you for watching and I'll see you soon on a TV screen somewhere."
Walker took part in the most recent series of the BBC's Strictly Come Dancing, finishing in fifth place with partner Nadiya Bychkova.
Walker's departure from the BBC's flagship breakfast show comes hot off the heels of a string of other high profiles moves away from the corporation.
Fellow BBC journalists Emily Maitlis and Jon Sopel are leaving to launch a new podcast and host a radio show together on LBC.
Film critic Mark Kermode and radio presenter Simon Mayo are also leaving their long-running Friday film review programme on BBC Radio 5 Live.
They will be doing a new twice weekly film and TV podcast series for Sony from the beginning of May.
Former England and Liverpool star Peter Crouch is also moving from 5 Live, taking That Peter Crouch Podcast to Acast.
Andrew Marr announced in November that he was leaving the BBC after 21 years to "get my own voice back."
He joined the broadcaster as political editor in 2000 and has presented a Sunday morning programme since 2005. Laura Kuenssberg will take over that slot later this year.
Marr will host an "opinion-led programme" on radio station LBC, a new show on Classic FM, and write for newspapers.