The 17-year-old from Essex was crowned the winner at the end of a live two-hour show on Sunday night (13 December).
Louisa, who was mentored by Rita Ora, burst into tears as hosts Caroline Flack and Olly Murs revealed she was the winner, leaving Reggie 'N' Bollie as runners-up.
An ecstatic Rita, who marked her first win in her first series as a judge, praised Louisa on stage: "Thank you to everyone who voted and thank you to Louisa for being the most talented, hardworking and gorgeous winner!"
Louisa, who stunned fans from her very first audition in the series, was overcome with emotion as news of her win sunk in.
"I've imagined this day for so many years," she said. "You have no idea how much this means to me, thank you so much."
Louisa then performed her brand new debut single, a cover of Bob Dylan's 'Forever Young'.
Recap the weekend's final here:
Saturday's show saw Louisa, Che Chesterman and Reggie 'N' Bollie take to the stage as the three finalists.
Louisa sang 'Fighter' by Christina Aguilera and 'I Believe I Can Fly' by R Kelly.
Che sang 'In The Air Tonight' by Phil Collins and 'Valerie' by Mark Ronson featuring Amy Winehouse.
Reggie 'N' Bollie sang 'Jump' by Kriss Kross and 'Spice Up Your Life' and 'Boom Boom Boom' by Spice Girls and Outhere Brothers.
Then it was time for the musical duets. Louisa performed with her mentor Rita, Reggie 'N' Bollie sang with Fuse ODG and Craig David, while Che duetted with Rod Stewart.
Guest performances saw two collaborations - Ben Haenow and Leona Lewis, and Fleur East and Little Mix.
Before the end of the show, it was revealed that one contestant would be leaving the competition, and Olly and Caroline announced that Che - who Simon Cowell had called the 'underdog' - had received the fewest public votes and so finished in a respectable third place.
Praising his mentor Nick Grimshaw, Che said: "Its been an amazing journey. Meeting Nick has been a highlight. I think we'll stay friends for a long time."
Sunday's show saw all of this year's contestants return to the stage for a final group performance of Macklemore & Ryan Lewis' version of 'Downtown'.
Joining Louisa, Che and Reggie 'N' Bollie was 4th Impact, Max Stone, Seann Miley Moore, Lauren Murray, 4th Impact, Alien Uncovered, Bupsi, Anton Stephans, Kiera Weathers, Mason Noise and Monica Michael.
With just Louisa and Reggie 'N' Bollie battling it out for the crown, they both had to choose their song of the series.
Reggie 'N' Bollie did their mash up of 'What Makes You Beautiful', 'Cheerleader' and 'Move It' while Louisa took on 'It's A Man's World' once again.
The two acts also performed their versions of the winner's single, a cover of Bob Dylan's 'Forever Young'.
Coldplay, One Direction and Adele were the guest performers of the night.
After what has been an eventful series, Olly and Caroline gathered Louisa and Reggie 'N' Bollie on stage to deliver the final verdict.
Drum roll... Louisa is the winner of The X Factor 2015!
Louisa's version of 'Forever Young' is available for download now. The CD, which is due for release on Wednesday 16 December, will include recordings of 'God Only Knows' and 'Let It Go' - two songs which Louisa performed on the show.
'Forever Young' will be in aid of children's charities, Together for Short Lives and Shooting Star Chase. 100% of profits from the sale of each CD and download will be shared equally by the two charities.
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