Baines, who has missed Everton's last six games due to a toe injury, was the subject of several failed bids by Manchester United during the summer.
Speculation has suggested that former Everton boss David Moyes will make a fresh attempt to take the 29-year-old to Old Trafford when the window reopens.
However, Martinez has no intention of letting Baines leave Goodison Park and insists the England international is 'vital' for the second half of the season.
"In football you don't work long term, you go window to window and I can guarantee that Leighton will be here at the end of the season," said Martinez. "That is 100 per cent.
"That is how you work from window to window - to have a stronger squad than previously.
"Leighton is vital for us in the second half of the season. Bryan Oviedo has been terrific, but we need Leighton and Bryan fit for us, if we are going to fight for our football dream."
One player who could be on his way out of Everton in January, though, is Croatia international striker Nikica Jelavic, who has been the subject of enquiries from two clubs.
Speaking ahead of Sunday's game against Southampton, Martinez told evertontv: "We've had a couple of enquiries from two different clubs for one of our players.
"Nikica Jelavic is someone who we have made it very clear to - we've had a conversation and he has the World Cup around the corner.
"He needs to be playing minutes and we understand that. We'll always try to give it a good assessment of the situation but that's not affecting anyone at the club.
"All we are focusing on is being as good as we can on Sunday.
"Whatever happens in January will be for the good of the football club and that's where the work is."