The new deal keeps the 24-year-old Ireland international at Goodison Park until the summer of 2018.
"I'm over the moon. I'm glad to get it all done and to sign for so long at a club like Everton is a great achievement - I want to be here for as long as possible," he told the club's website.
"Signing a contract here is fantastic and I'm really looking forward to the next couple of years.
"I'm going to keep working as hard as I have done since day one and keep improving every day in training to impress the manager.
"I'm really happy to have signed for the length of time that I have."
Boss David Moyes said: "He deserves it as he's worked really hard since the early days when we brought him in from Ireland.
"Seamus is one of the young boys. He is improving. He came as a right-back and we've used him wide right and he has got his chance to settle in and I feel I can use him in either position whenever needed.
"He's a long way to go to get to Leighton Baines' (who has been a permanent fixture on Everton's left for years) standard at the moment but that's why we've kept him for a long time because we think there's a chance of good improvement in him."
Coleman has missed the last three games through injury but has proved to be an increasingly useful addition on the right-hand side.
Having joined from Sligo Rovers in January 2009 for just £60,000 he made his debut against Benfica in October that year and later went on loan to Blackpool in their promotion-winning year.
He was rewarded with a new three-year contract in May 2010 and signed an improved four-and-a-half-year deal in January 2011 in a season in which he made 24 Premier League starts, his best to date.
Moyes found a place for him on the right of midfield for many of those early first-team appearances but has since slotted him back into the side on the right of defence.