The Irish singer, who is a judge on the reality TV show alongside Guy Sebastian, Natalie Bassingthwaighte and Mel B, said that he thinks Ruffo has more potential than his fellow finalists Andrew Wishart and Reece Mastin.
Keating wrote on his Twitter page: "After everything that we have seen as judges on #xfactorau I think now I would like Johnny to win. He has work to do on vocals but potential. (sic)"
He reminded his followers: "Remember it's called the #xfactorau it's not just about great singing if that was the case Audio Vixen would still be here. It's about X. I do agree Reece is great but not sure he has all the elements to win."
Keating described Wishart's performance last night as "awesome", but then wrote on Twitter: "Go Johnny."
The judge and mentor has previously shown his support for Mastin, even controversially telling the teenager that he should consider leaving school in order to pursue his music dream.
Three Wishez - Keating's last group in the competition - were eliminated by public vote last week.
> Aussie 'X Factor's Reece Mastin loses voice just before grand final
> Australian 'X Factor's Andrew Wishart: 'I miss my family'
The X Factor Australia final airs tonight (Tuesday) at 7.30pm on the Seven Network.
Watch Johnny Ruffo perform during the X Factor final below: