With fellow forward Morgan Smithies leaving for Perth next season and veteran prop Josh Papali'i also departing at the end of this campaign, there is concern the Raiders will be without quality middles next year.
Rumours are circulating that out-of-favour front-rower Corey Horsburgh is also set to move to the Bears, meaning Mariota's deal will provide immediate relief for the Green Machine.
Coach Ricky Stuart has previously lavished praise on the 24-year-old, saying he will be integral to the Raiders' future, having featured in 87 NRL games for the club.
Mariota has played a big role this year, with the squad decimated by injuries.
He has had to settle for either a second-row appearance or playing from the bench until this Saturday's clash with Cronulla, where he will line up in his usual starting prop jersey for the second time since round seven.
"I'm so happy to re-sign here. Canberra is home now, and this club has given me everything. I just want to repay that as best I can," Mariota said in a statement.
"I'm so lucky to be learning off Papa (Papali'i) and Taps (Joseph Tapine). I just pick their brains and take anything I can out of their games into mine."
Raiders recruitment manager Chris Hutchison said Mariota had become an important part of the team, having come through the club's pathway system.
"He is a hard-working, dynamic forward who has developed into an international player and will continue to grow at the Raiders," Hutchison said.
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