The 36-year-old made 55 appearances for Scotland and replaces Alan Archibald as Partick manager. The 'Jags' currently sit eighth in the Scottish Championship.
Caldwell spoke to the club's official website ahead of Partick's game with Alloa on Saturday.
He remarked: "Right from my first interaction with the club, I have been impressed with how the organisation is run.
"It very quickly became clear to me that there are positive and exciting plans in place, backed up by a strong structure from top to bottom.
"There is a huge amount of potential in this squad. But it won't happen overnight. We can't do it on our own either, I need support from the fans and so do the players. If we are together, I have no doubt we can achieve a massive amount."