The 40-year-old has been ruled out for four months with an arthritic foot problem, therefore preventing him from qualifying for the showdown against the USA.
"Everyone who knows me knows how much the Ryder Cup means, having given me memories to savour over the past 12 years and created friendships that will last a lifetime," said Poulter. "Therefore when it became clear, given my current foot condition, that I would not be in a position to try and qualify for the team for a sixth time this year or indeed be able to justify a wild card pick, I was truly honoured and delighted to be asked by Darren to be a vice-captain.
"The best moments of my career have come when I have been wearing the blue and gold crest of Europe on my chest, knowing at the same time that the contest means so much to every European player and to every European fan, either in the crowd or watching back home on television. Darren shares that same passion and he knows I will be there to assist him and the 12 players who will line up at Hazeltine in any way I can.
"We have a tremendous record in the Ryder Cup in recent times, having played with passion, desire and commitment. It is something we should be proud of and, rest assured, I will do everything in my power to maintain that intensity and help Europe's quest to retain the trophy."
Clarke added: "I am absolutely delighted to name Ian as my fourth vice-captain for the match at Hazeltine. Ian's Ryder Cup record speaks for itself and stands comparison with anyone in the history of this great contest. It goes without saying what a vital presence he will be in our team room come September.
"I picked Ian as a wild card for Europe's EurAsia Cup team in January and while he delivered on the golf course with three wins out of three, I also saw first-hand his influence off the golf course, especially amongst the younger players who were experiencing team match play golf for the first time. When he spoke, everyone listened, and the same will be the case at Hazeltine."
Poulter has sealed 12 wins and two half-points in 18 Ryder Cup appearances.