The rock star was caught on television crying at Celtic Park after the Scottish champions beat mighty Barcelona in the Champions League on Wednesday.
There had been a late scare when Barca star Lionel Messi scored in the 90th minute, but Celtic held on to mark its 125th anniversary with a memorable 2-1 win.
The victory puts Celtic on course to advance from the group stage to the knockout phase. Neil Lennon's side is two points behind Group G leader Barca.
Stewart is one Celtic's most devoted, and famous, supporters and makes mention of his love for the club in his hit song "You're in My Heart."