Howson, 25, joined the Canaries from Leeds in January 2012 and has made 60 appearances, scoring five times including a fine individual effort on the final day of last season at Manchester City.
"As soon as I found out there was an offer there I was very interested in getting it signed. I am very settled here and happy to sign," Howson said on the club's official website, www.canaries.co.uk.
"I have really enjoyed the season so far and have had a good run in the side. I just want that to continue, and obviously staying here longer gives me the chance to do that."
Chris Hughton saidon the club's official website :"A new contract comes in line with Jonny's form this season, which has seen him be a mainstay of the team and put in some impressive performances.
"In the period of time that Jonny's been here, there's been progression in his influence in the team and in the matches, and we're delighted to be able to secure his services for the next four-and-a-half years."