The group fended off stiff competition from One Direction's Louis Tomlinson, Little Mix, Rag'n'Bone Man and an old festive favourite, 'All I Want For Christmas Is You' by Mariah Carey, to score the prestigious title, the Official Charts Company said.
Matt Terry, winner of 'The X Factor', fell to number eight with his debut solo single 'When Christmas Comes Around', after the Ed Sheeran-penned track debuted in the charts last week at number three.
Clean Bandit told OfficialCharts.com: "To have been number one for seven weeks is something amazing in its own right that we are all incredibly proud of, but to now be Christmas number one is mind-blowing."It's something we never imagined would happen with 'Rockabye' when we were writing and recording it."
The band's Grace Chatto added: "We did think about making a Christmas version but there wasn't any need!"
Guitarist and keyboard player Jack Patterson added that his mother had a "theory" about 'Rockabye', in that the lyrics refer to Jesus Christ.
He said: "It's weird actually, my mum has got this theory about the song, about (the lyrics) 'Rockabye, baby' being about Jesus, and that Jesus is the baby.
"She's talking about this woman who's looking after the child on her own and saying it's a bit like Mary - she had the Immaculate Conception - and Joseph is kind of there ... but not."
Rockabye has not only achieved the status of Christmas number one but the catchy track has also become one of 2016's biggest hits across downloads and streams with 589,000 combined chart sales.
Brits Critics' Choice Award winner Rag'n'Bone Man's single 'Human' has finished the race for the chart top spot in second place, while Tomlinson's collaboration with Steve Aoki, 'Just Hold On', ended the week at number three.
Little Mix's 'Touch' climbed from last week's position of 23 to number four and Carey's classic holiday hit from 1994 completes the top five.
The musical effort from Friends Of Jo Cox - 'You Can't Always Get What You Want' in memory of the late Labour politician - is at number 136 in the final chart before Christmas Day.
A team of volunteers and staff from hospices across London, the London Hospices Choir, made their chart debut with their cover of Mike & The Mechanics' 'The Living Years' at number 81.
Following a strong campaign push from Rangers FC fans to get The Dave Clark Five's 'Glad All Over' to number one, the single eventually made it into the top 40 at number 31.
Well-known Christmas songs 'Fairytale Of New York' by The Pogues and Kirsty MacColl and Wham!'s 'Last Christmas' have enjoyed a resurgence in the charts, coming in at number 15 and number 16 respectively.
Over on the Official Albums Chart, Michael Ball and Alfie Boe's collaborative album 'Together' has also retained its prime position to become this year's Christmas number one album.
The singers said: "It's a dream come true. There's only one thing better than having a number one album and that's having a Christmas number one album."
'Together' has been in the charts since its release in November and last week peaked in the top spot.
Little Mix's 'Glory Days' is at number two, Elvis Presley's 'The Wonder Of You' is at number three, and the Rolling Stones' 'Blue & Lonesome' and Olly Murs's '24 Hrs' are fourth and fifth respectively.