The 20-year-old - who performed a duet with Beyonce Knowles - got 1 million more votes than runners-up JLS.
Eoghan Quigg took third place on the The X Factor final last night.
It was a 2nd chance for Alexandra, 20, because she was eliminated from X Factor 2005 by Louis Walsh before the live shows.
JLS were disappointed to be beaten into 2nd place but still have a great future ahead of them.
Cute Eoghan Quigg, 16, also has a huge fan teenage fan base.
As winner of The X Factor, Alexandra scoops a £1m record deal and will release her stunning version of Leonard Cohen's Hallelujah, a cert for the Christmas No 1 slot.
Alexandra was so overwhelmed by her win that she fell to the floor in front of host Dermot O'Leary, speechless with emotion.
Finally, Alex composed herself so that she could sing Hallelujah as X Factor winner 2008. But her voice broke halfway through the song because she was still so choked.
Watch Alexandra singing with Beyonce below.
Click on the clip below to watch Alexandra singing the winner's song, Hallelujah.