The 24-year-old is the skipper at Counties Manukau and will arrive at the Kingspan Stadium in October after the conclusion of the ITM Cup.
The Kiwi will provide cover for injuries alongside the currently fit Franco van der Merwe, Lewis Stevenson and Neil McComb.
Raaymakers has represented the All Blacks at U20 level - as well as making two Super Rugby appearances for Auckland Blues in 2013 - and says he can't wait to get going.
"It has always been an ambition of mine to play overseas and when the opportunity came up to join Ulster I had no hesitation in saying yes," he said.
"I look forward to joining up with the squad next month and to playing my part in helping Ulster Rugby be successful this season."
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