The 25-year-old moved to Carrow Road from Championship club Leeds United in January 2012 and has since scored five goals in 60 appearances for the East Anglia outfit.
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He told the club's official website: "I'm very pleased. As soon as I found out there was an offer there I was very interested in getting it signed. I’m very settled here and happy to sign.
"I've 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."
Norwich manager Chris Hughton believes Howson's new deal is richly deserved.
He said: "We're delighted that Jonny has put pen to paper on a new deal.
"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."