The back-row forward will link up with the Exiles after Canada's World Cup campaign in New Zealand.
Sinclair, 25, has been playing for Canadian side Atlantic Rock and he was arguably his country's outstanding player during the Churchill Cup competition earlier this summer when they lost to England Saxons in the final.
Exiles head coach Toby Booth said: "I saw a lot of Jebb during the recent Churchill Cup, and I was impressed.
"His workrate around the park was of a high standard and allowed others around him to shine, and that is what you need from a forward in the Premiership.
"He offers another lineout option and he is a strong ball-carrier, which will further strengthen us for this season."
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.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
$29.99
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
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.
