The singer, who won the show in 2008, has been drafted in as a last minute replacement for Kelly Rowland, who will miss the show due to ill health.
Burke said: "I'm really excited to be a guest judge on The X Factor this evening. As someone who has been part of the show all the way to the final, I can't wait to share my opinions. I'll be there fighting for the girls tonight!"
A statement released yesterday confirmed that the Destiny's Child star is unable to fly back to the UK from LA due to a throat infection.
"Kelly is devastated she won't be here for this weekend's show," read the statement. "She is extremely ill with a viral throat infection and her doctors aren't permitting her to fly."
The news follows rumours that Rowland had flown to America after falling out with fellow judges Tulisa Contostavlos and Louis Walsh.
Contostavlos and Walsh accused Rowland's act Misha B of bullying fellow contestants on last week's show, prompting Rowland to warn Contostavlos to "leave things backstage".
Rowland has insisted that she hasn't abandoned her acts and has been in constant contact with them from the other side of the Atlantic. She had been due back in the UK on Thursday.
Burke will join Contostavlos, Walsh and Gary Barlow on the X Factor judging panel from 8pm tonight on ITV1.
Watch Alexandra Burke singing 'Candyman' on The X Factor in 2008 below: