Du Plessis has impressed since arriving at the club from Namibia on a short-term deal in January, as has towering lock Cannon following his return from injury after missing a large part of the season.
Director of rugby Dai Young said: "I have been really impressed with both of these players this season.
"Tinus is a very versatile back-row and he has got his head down and worked incredibly hard since arriving in January.
"With the hunger and desire he has shown he is fully deserving of a permanent deal.
"James Cannon is a player with enormous potential and he was unlucky to miss a lot of this season with an injury but since he's come back in he has been very impressive and I had no hesitation signing him up for next season."
Welcoming his new deal, Cannon said: "With a lot of talent here in the second-row I am really happy with a new contract.
"It has been a frustrating season with injury for me but I have received incredible support from our medical team and I am really grateful for them to be back on the pitch now.
"Starting last weekend was fantastic for me and I hope I can get a good run of games together now."
Du Plessis added: "I am delighted to have secured my future with Wasps and I am hugely grateful to Dai for this opportunity.
"I have been learning and improving all the time in the set-up since I arrived and I feel I have a lot more to give."
The pair are the third and fourth players to have signed contract extensions at Wasps this week, following in the footsteps of Hugo Southwell and Tim Payne.
Wasps have also signed centre Lee Thomas from Lyon and hooker Rhys Thomas from Cardiff Blues for next season.
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