Europe captain Nick Faldo made his announcement shortly after the end of the Johnnie Walker Championship.
"I picked Ian because he is obviously a very determined guy and I love his attitude and obviously what he did at the British Open," Faldo said. "The back nine he played there was with the intention to win. He had the emotional feeling and was not playing to finish second."
Poulter, who ended up second behind Padraig Harrington at the British Open, could have finished in the top 10 and earned an automatic berth at the Johnnie Walker Championship, but he skipped the event.
"Maybe I should have gone to play Gleneagles, but I stuck by my word," Poulter said from the Deutsche Bank Championship, where he missed the cut. "Looking back at the time I made the right decision for me, but it was so difficult to choose the right thing to do and it's obviously been a very difficult decision to try to get across."
Oliver Wilson clinched the 10th and final automatic spot by finishing in a tie for 10th place at Gleneagles on Sunday, while Justin Rose and Soren Hansen also qualified to make their debuts at the competition.
Harrington, Sergio Garcia, Lee Westwood, Henrik Stenson, Robert Karlsson, Miguel Angel Jimenez and Graeme McDowell had already made the team.
Darren Clarke, Colin Montgomerie and Carl Petterson were among the players hoping to get a wild-card selection.
"Nick rang me after my last round at Gleneagles to say that I wasn't going to be one of his picks and I simply wished him all the best," Clarke said in a statement. "Obviously I am disappointed that I wasn't selected, but I have dealt with much worse and it will make me all the more determined to make the team that will play in Wales next time."
Clarke's wife Heather died of cancer in 2006, six weeks before he was selected to play in that year's Ryder Cup at The K Club. He won all three of his matches there.
"I fully respect the captain's decision and I will be supporting the team from wherever I am," said Clarke, who has played on the last five European teams.
United States captain Paul Azinger said he was surprised by Faldo's picks.
"I thought it was between Casey and Poulter. I thought Clarke was a lock," Azinger said. "But the guys he picked are really good. His dilemma is a nice one to have. You look at all the guys he had to choose... I don't have that. It's not like I have a bunch of veterans with monster records at the Ryder Cup."
Faldo said he had not yet informed Casey of his selection because the Englishman was on a plane back to Europe.
"Tee to green, Paul has been playing absolutely the best through the summer and through the majors, and I was kind of waiting for his putting to turn around," Faldo said. "In the last few weeks he's been very close to being No. 1 in putting averages each week. So that's been very important."
He added that other factors in choosing Casey were his win in the 2006 World Match Play Championship and his good record in Ryder and Walker Cups.
"I feel very good about those two picks," Faldo said.
Nick Dougherty would have had a chance to make the team by winning the Johnnie Walker Championship, but his 69 left him in seventh place.
AP Golf Writer Doug Ferguson in Norton, Mass., contributed to this report.