Team Europe announced the move on Tuesday. Harrington, 47, is a six-time Ryder Cup participant. He played on four winning teams at the tournament. Harrington was an assistant captain in 2014, 2016 and 2018.
The 2020 Ryder Cup will be played from September 25 through September 27 at Whistling Straits in Wisconsin.
"I have played under many wonderful European captains since I made my debut 20 years ago and I would like to think that my captaincy will be a mix of all of them," Harrington told reporters. "I'm very much looking forward to taking on this role."
Team Europe beat Team USA 17.5-10.5 at the 2018 Ryder Cup at Le Golf National in Guyancourt, southwest of Paris.
"It's an away match and I have to be part of this and try to find an edge to try and get the most out of their abilities and hopefully get a win,'' Harrington said. "It's a different league, a different level. I really feel conscious that I have to find that edge. And I don't take it lightly. It's something I will take on over the next 18 months.''