Yesterday afternoon (October 20), the 'Candy' singer - who is currently in Europe promoting his forthcoming album Take The Crown - tweeted "TEAM RYLAN ALL THE WAY".
"VOTE RYLAN," he added in a short blog on his official website.
Williams's support for Clark will likely annoy his bandmate and X Factor judge Gary Barlow, who has frequently voiced his dislike of the Essex-born contestant.
Over the last two shows, Barlow has watched as Melanie Masson and Carolynne Poole from his over 28s category were sent home, while Clark remained in the competition.
He has described Clark's performances as "embarrassing" and told the other panellists they should be ashamed of having put him through to the live stages.
It has even been reported that Barlow has told his remaining hopefuls, Kye Sones and wildcard act Christopher Maloney, to stay away from the 23-year-old after branding him a bad influence with a "bad attitude".
Last night, the remaining contestants performed songs that came under the theme 'Club Classics'. Clark sang a mash-up of Jennifer Lopez's 'On the Floor', Groove Armada's 'I See You Baby' and Rihanna's 'Please Don't Stop the Music', which Barlow said was the worst performance of the night.
