The 24-year-old former Northampton Saints full-back signed in the summer of 2012.
He went on to become one of the club's most consistent performers, starting all but two possible matches, before winning his first senior cap on Scotland's 2013 summer tour of South Africa.
Tonks said: "I came to Edinburgh Rugby intent on playing as much as possible and hoping to pursue an international career with Scotland so, on a personal level, my time here has gone really well.
"I was happy to achieve my goals in a pretty whirlwind year, so it was an obvious decision for me to try and stay as long as possible, even before the club opened the talks to extend my contract.
"Naturally I'm keen to get more game time for Scotland after getting my first cap but it's really important to me to have a good season with Edinburgh Rugby.
"As a team we need to perform better. There's a lot happening behind the scenes to get excited about and hopefully this time next year we'll be looking back on a better year."
Managing director David Davies added: "We have been very impressed with Greig.
"He had a very good debut season at the club and we were delighted to watch him get his first Scotland cap in the summer.
"He's a genuine talent and individual, and the kind of bright young player that we want to develop at this club."
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