The 20-year-old has been linked with some of the Barclays Premier League's biggest sides this summer after an impressive season in 2010/11, but the Latics have made hanging on to one of their biggest assets a priority.
Martinez said of McCarthy, who joined Wigan two years ago from Hamilton on a five-year contract: 'James is a big diamond of ours. I felt he had a tremendous season last campaign and he deserves a reward.
A real gem: Martinez is keen to tie McCarthy down to a new deal
'We've got another five or six players that we're going to look to try to reward and give them new contracts.
'We're still in talks, nothing has been finalised, but it's not a real secret that we want to reward the players who have been performing well and developing with us, and James McCarthy is in that group.'
Meanwhile, Martinez dismissed reports of a rift with striker Hugo Rodallega, who is out of contract at the end of the season.
The Colombian, whose goal at Stoke on the final of last season preserved the Latics' Premier League status, is another to have been linked with a move away from the DW Stadium, but Martinez insisted tying him down to a new deal is not a major concern.
No rift: Martinez insists he has not fallen out with Hugo Rodallega
The striker was only fit enough for the bench in last weekend's draw with Norwich after returning late following his involvement in the Copa America, but he could return to the starting line-up at Swansea on Saturday.
'We've never had a problem with Hugo,' added Martinez. 'I know there's been a few things in the press, I don't know where they've come from. We would love to have Hugo here for the long term but at the moment it's not a priority.
'His agent's in Colombia, we've got a very good relationship, and we'll see what develops from there. But my focus on Hugo is for him to be as fit as he can be because when he's on top form he's a unique player and an important player.'
CLICK HERE TO SIGN UP FOR OUR FREE FANTASY FOOTBALL GAMEIn-demand McCarthy pledges his future to Wigan with new five-year dealMartinez confident of keeping his star players at Wigan after avoiding relegationAll the latest Wigan news, features and opinion