Gidley, 31, arrived at the club at the beginning of 2007 from Newcastle Knights, and has scored 37 tries in 84 appearances for Saints.
“He is also a great clubman and a very positive influence on the promising young outside backs who are rapidly establishing themselves as first team regulars. It is good news all around.”
He said: “I have more than enjoyed my three seasons at St Helens and it is great to commit to a fourth.
“We have a fantastic blend of experience and talented youth and are very well placed to continue our record of success in Super League in the years ahead.”
Saints host Celtic on Friday looking to consolidate their four point lead at the top of Super League.