It's huge news in the world of entertainment shows, not only because Tonioli spent many years at rival broadcaster BBC, but also because the BGT judging panel had been unchanged in a decade.
Comedian and children's author Walliams leaves after insulting comments he made about contestants in 2020 emerged from an audio recording, while Tonioli is free to join ITV since departing Strictly after Covid restrictions interrupted his long-running role.
BGT shared a photo of the 67-year-old Italian dancer and choreographer in his new seat on the panel on Tuesday, as Tonioli said: "Here we are, Britain’s Got Talent. I am so excited. It’s my first show, I have no idea what’s going to happen, but I’m sure it’s going to be brilliant."
All the Britain's Got Talent judging line-ups
Simon Cowell, Amanda Holden, Piers Morgan - 2007 - 2010
Cowell and Holden have been around since the early days of Britain's Got Talent, but for the first four series they were joined by controversial broadcaster Piers Morgan.
Despite the risk of that being too much Mr Nasty for one judging line-up, it worked strangely well while it lasted - until Morgan left to launch his US TV career.
Read more: Britain's Got Talent's 10 biggest scandals
Back in those days, there were only three seats on the panel, although Kelly Brook was brought in as an extra judge for series three.
However, less than a week after she began, show bosses decided a fourth judge overcomplicated things and so she was credited as a guest judge instead.
Cowell's old X Factor pal Louis Walsh also debuted as a guest judge in series four when Cowell fell ill and couldn't attend some of the auditions.
Amanda Holden, David Hasselhoff, Michael McIntyre - 2011
The show's first big line-up change came in series five, with Morgan off to break America and Cowell hot on his heels launching The X Factor USA.
Holden was the only original panel member left holding the fort for the whole series, although Cowell did manage to return as a fourth judge in time for the live shows.
In the meantime, Holden was joined by Baywatch star David Hasselhoff and comedian Michael McIntyre, although the combo was shortlived as both new signings didn't want to come back for series six.
Walsh was a guest judge again during one audition round, covering for Hasselhoff who was performing in a pantomime.
Simon Cowell, Amanda Holden, Alesha Dixon, David Walliams - 2012 - 2022
So began the longest running line-up which lasted from series six of Britain's Got Talent up to 2022.
Cowell returned full-time to oversee the panel, Holden stayed on in her usual seat, and the show recruited Dixon and Walliams to join them.
Dixon had been a judge on BBC rival Strictly Come Dancing but was lured to ITV by Cowell, while Walliams rounded out the panel, making what had seemed like a strange job for a comedian when McIntyre took it on fit perfectly.
However, Walliams' reputation as friend of the contestants appeared to come crashing down in November 2022 when The Guardian uncovered a leaked audio recording from the 2020 auditions in which he was heard making some very derogatory remarks about contestants.
Walliams apologised, saying: "I would like to apologise to the people I made disrespectful comments about during breaks in filming for Britain’s Got Talent in 2020.
"These were private conversations and — like most conversations with friends — were never intended to be shared. Nevertheless, I am sorry."
But the damage was done, and after rumours that Alan Carr would replace him, the latest judge was eventually confirmed as Tonioli.
There were a few guest judge additions over the years - in series six, Carmen Electra covered for Holden for some of the auditions as she recovered from her daughter's birth.
Cowell has also had a few stand-ins, including hosts Ant and Dec covering a day of auditions for him in series eight, Walliams' mum Kathleen who was in the audience filling in when he was late to work in series 10, and former BGT winner Diversity star Ashley Banjo replacing him during series 14 after his electric bike accident.
Watch: Ant McPartlin needed medical attention on Britain's Got Talent set