Saints manager Derek McInnes played with the 31-year-old at West Bromwich Albion and is delighted with the capture of the man who finished up his second spell at Carlisle last month.
"I know Scott from our time at The Hawthorns and have kept an eye on his career since then," he told the club's official website.
"He will complement our existing options - he has pace and strength, can play in a wide role if required and I am sure he will make an impact and hopefully enjoy his football at the club."
Dobie expressed his delighted with the move, saying: "I'm really looking forward to it.
"I've never played in the SPL but I have a lot of experience in England and I have faith in my own ability to get goals for St Johnstone."