It's believed the Czech Repubic international's new deal will be a rolling one-year contract at the Emirates.
The 31-year-old has been in good form recently, scoring twice in three games, against Tottenham and AC Milan.
Commenting on his new deal, Tomas said: “It's a great honour to sign a new deal with Arsenal, I love the Club and am proud to wear the Arsenal shirt. It's felt like home since I arrived six years ago and I couldn't be happier to commit my future to the team.
"The manager, my team-mates, the staff and the supporters - everyone plays their part in making this a great club, and I feel privileged to be a part of it. I believe we have a really talented group of players and, together with the manager, the future looks bright. It's always been my goal to win trophies here at Arsenal, and I will fight hard to achieve my dreams here."
Manager Arsène Wenger added: “I’ve always been a big admirer of Tomas’ abilities and am delighted he’s chosen to commit his future to the Club. I believe he is an exceptional player, a natural footballer who is technically top class.
"He plays the style of football we champion here and shares completely our philosophy. He is always ready to work for the sake of the Club, always willing to put the team first. I have huge respect for him.”