Cusiter has 30 European appearances to his name from his time at Border Reivers, Perpignan and the Warriors and he is looking forward to three more years in Glasgow.
"I'm delighted to have signed for another three years, and I'm genuinely excited about what we're in a position to realise here during that period of time," said Cusiter.
"I'm personally very ambitious: I want to perform to a very high level with the Warriors in the PRO12 and Heineken Cup, helping the team achieve success, and I'm also determined to establish myself as the first choice scrum-half for Scotland.
"When I came to think about my future, my priority was making sure I was at a club that would help me reach those goals.
"I had a number of positive conversations with [Warriors head coach] Sean Lineen, [Scottish Rugby director of performance rugby] Graham Lowe and [Scottish Rugby chief executive] Mark Dodson, and was energised by what they had to say and by the challenge that lies ahead for us all.
"The vision is for the Warriors to become a regularly winning team in both the PRO12 and Heineken Cup, and to become a real focal point for rugby in Glasgow and the west of Scotland.
"It hasn't gone unnoticed among the squad that this ambition is already being backed up with action: contracting is being done early in the season, we've brought in a lot of new sponsors, we're moving to a new stadium, and we've just added a quality international winger to the team in David Lemi.
"All of that, and the fact I really enjoy Glasgow as a city, combined to help make up my mind that this is where I want to be for the foreseeable future."