Naismith, who joined Everton from Rangers in 2012, will remain at Goodison Park until 2019.
The 28-year-old has been in good form for Roberto Martinez's side this season, scoring five Premier League goals to help Everton up to 10th in the table.
"I am so happy here. From the moment I signed my first contract here it wasn't long before I saw what a great club it was," Naismith told EvertonTV.
"Over the months and years it has progressed into a fantastic move for myself on and off the park. I am delighted to have agreed an extension, which will keep me here for a few more years yet.
"When you come to the last 18 months of your contract, negotiations start happening and it was pretty relaxed and straightforward because I have loved my time here and this season and last season you have seen the best of me; hopefully there's more to come.
"I think I have felt more wanted in recent times than at the start of my Everton career. Confidence has grown, I have put in some good performances and we have had a successful time as a team as well.
"I would say it is probably the best form of my career. I understand the game more, I understand what the manager is wanting from me and I have played in a few different roles and contributed in those roles. It's definitely the most consistent I have been."
Naismith has scored a total of 19 goals in 86 appearances in all competitions for Everton since swapping Glasgow for Merseyside after Rangers were liquidated in 2012.
