The appointment of the three-time major champion, 47, was announced in a news conference at Wentworth on Tuesday.
Harrington succeeds Thomas Bjorn, having served as a vice-captain as the Dane oversaw an emphatic European success at Le Golf National in September.
A winner in four of his six Ryder Cup appearances as a player, Harrington was also a vice-captain in 2014 and 2016 under Paul McGinley and Darren Clarke.
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