The 22-year-old, who can play full-back, fly-half and centre, has scored seven tries in 19 appearances for Saints since joining from the Leicester Tigers academy in 2010.
Pretoria-born Tonks has represented England at Under-16, Under-18 and Under-20 level, but is eligible for Scotland through his Ayrshire-born mother and has agreed a deal at Murrayfield through to the summer of 2014.
Edinburgh head coach Michael Bradley said: "We're very happy to have secured the services of Greig. His age profile is very good and he's a brave, skilful, aggressive player and is a quality striker of a rugby ball.
"His versatility is a huge plus. Another standout characteristic is that he's an extremely driven player with a hunger for success that sits well with the ambitions of this club."
Tonks said: "This move presents a pretty awesome opportunity for me to develop as a player and be part of something special with Edinburgh."
Tonks is the latest player to join Edinburgh since South African prop WP Nel signed from Super 15 side Free State Cheetahs, while 13 other leading players, including winger Tim Visser and Scotland internationals Greig Laidlaw, David Denton and Lee Jones, have recently committed their futures to the club.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
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The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.