The singer-songwriter, who won season 11 of the Fox talent contest, will perform new single 'Gone, Gone, Gone' on Thursday, March 14's results show.
Phillips joins Bon Jovi, whose American Idol guest appearance was announced last week.
It has also been announced that 'Gone, Gone, Gone' will be this year's contestant 'farewell' track, playing out at the end of each results show. Last year's farewell song - as recorded by season 10 winner Scotty McCreery - was a cover of Tim McGraw's 'Please Remember Me'.
This season on American Idol, the Top 20 hopefuls must in effect 'qualify' for the final run of live shows by singing to win the general public's votes.
Ten contestants each will perform live from Las Vegas on Tuesday, March 5 and Wednesday, March 6, and the results of the first public vote will air from Los Angeles on Thursday, March 7.
American Idol continues tomorrow (February 28) at 8/7c on Fox in the US. The show airs on Thursdays and Fridays at 8pm on 5*.
Watch Phillip Phillips's music video for 'Gone, Gone, Gone' below:
'American Idol': Meet season 12's Top 20 contestants - photo gallery:
Copyright: Fox Michael Becker