The news follows last week's commitment of Cale Morton, which was also for two years.
Melbourne general manager of list management Tim Harrington said securing Morton and now Grimes and Martin was another step in the right direction for the rebuilding club.
"What we want to try and establish at the club is that there is never any doubt in our players wanting to stay," he told melbournefc.com.au.
"And re-signing pretty much becomes a formality, because the players are very much aware of a building momentum in the club that will provide success down the track.
"Every time they run out on the training track and every time they run out on the ground, they look around and say: 'Have a look at the talent we've got here – we are going places'.
"You want to be a part of that."
Grimes, Melbourne's second pick (No. 14 overall) in the 2007 NAB AFL Draft, was selected after the trade that sent Travis Johnstone to the Brisbane Lions.
After struggling with a back injury for most of 2008, Grimes made his AFL debut in round 22 and had to recover from a recurrence this year before returning in round seven.
The half-back has since played 11 matches, although he has missed the past two with a hip flexor injury and could struggle to play again in 2009.
He was regularly among the Demons' better performers when in the side and has been touted as a long-term leader at the club.
Harrington said Grimes was further advanced than his experience suggested.
"Jack, within time, you would think will become a bona fide midfielder," he said.
"He can play multiple roles, he's a very composed footy player and thing that stands out about Jack at the moment is that he has hasn't yet played 20 games, but he plays like a 50-game player already."
Martin, who was recruited from Sandringham Zebras at No. 3 in the 2008 NAB AFL Pre-Season Draft, has already become one of Melbourne's most versatile players.
Despite a limited football background – he was an outstanding basketball prospect – Martin has been used as a key defender, key forward and in the ruck.
He has now played 26 matches after debuting in round 14 last year and Harrington said Martin's continual progress was exciting for the club.
"Stef is still developing as a player. So his potential for improvement is high, given he was reasonably late to football and he didn't play nearly the amount of football of what most young kids that come through the development pathway do," he said.
"The fact that he is a taller type and can play a multiple positions … we see that as a real strength.
"He's shown versatility obviously, but he's shown some attributes where you think: 'Wow, this kid could really bring something left of centre to the team'."
Harrington, who joined the Demons' football department from North Melbourne at the end of last year, expects more players to re-sign in the near future.
