The 34-year-old was available outside the transfer window after his contract at Wycombe was cancelled.
Duberry, who won the League Cup, European Cup Winners' Cup and UEFA Super Cup with Chelsea, revealed former Stamford Bridge team-mate Jody Morris was instrumental in convincing him to make the long trek north of the border.
"To be honest, I thought I had done all the travelling I was going to be doing in my career," he told his new club's official website.
"I'm very family-orientated and the arrangement with Wycombe suited me in that respect.
"However, I just want to play football, and when myself and Wycombe agreed to part company, Jody encouraged me to join St Johnstone.
"Jody and I have been close friends for many years and I know that when he went through a similar period of not knowing where his future lay, St Johnstone gave him a home and I know how content he is up here.
"That's had a big influence on my decision to join Saints.
"I'm joining the club with the blessing of my family and I am raring to go."
Manager Derek McInnes said: "He's driven seven and a half hours overnight to join up with us today (Friday) and I'm delighted with that.
"He is full of enthusiasm and made an immediate impression at training this morning.
"I'm delighted to get him to the club. He has the experience I have been searching for, he's won trophies at the top level and I want to get the best out of him.
"I've said to him that he should make sure that he makes the most of the latter stages of his career so that he has no regrets or frustrations when his career ends.
"I've told him to enjoy it, be stimulated by it - he's never played in Scotland before so it is another thing to discover and learn at this stage of a great career.
"He's joining a group of lads who have the right ethic and will help him do that."