Woolf has spent the last five years in charge of Tonga, leading them to a World Cup semi-final berth in 2017, and was interim coach of Newcastle for the last two NRL rounds.
“I’m excited and privileged to be joining St Helens as Head Coach,” he told saintsrlfc.com
“It’s something I have always wanted to do and it’s a great opportunity for me.
“St Helens is a club with an amazing history and I’m confident we can achieve great things together.”
Woolf will officially join the Saints in November and the club will hold a press conference to introduce him later in the year.
Saints chairman Eamonn McManus said: “We consider that Kristian is ideally qualified and placed to build upon the current success of the team and the club.
“He has the experience and expertise to bring the best out of our existing squad and to improve it further.
“We are very confident that the Saints will seriously compete for honours under his tenure and that we will continue to play attractive and exciting rugby league.”