Last year’s World Series champions, the St. Louis Cardinals, finished the regular season with an abysmal 83-78 record, but they didn’t have the worst winning percentage of a playoff team.
That distinction goes to the 1981 Kansas City Royals, who finished three games under .500 at 50-53. But this is misleading since ‘81 was a strike year, and baseball gave playoff berths to the winners of each half of the season. The Royals were 30-23 in the second half.
In a full season, no sub-.500 team has made the playoffs, but the 2005 San Diego Padres came close, winning the National League West with an 82-80 record, and nipping the 1973 New York Mets (82-79) in the race for worst post-season team.