Considering him the top defensive outfielder in their organization – including the major league club and all their minor league affiliates – the Tampa Bay Rays activated Kiermaier to both the 40- and 25-man rosters for the first time on September 30, 2013. He made his major league debut in that evening's wild card tie-breaker game – the Rays' 163rd game of the season – and played one inning against the Texas Rangers in the ninth as a defensive replacement. Rays general manager Andrew Friedman commented that the club included Kiermaier on the major league roster late that season specifically for his defense in center field. He played two innings in the 2013 American League Wild Card Game against the Cleveland Indians, which was his only postseason appearance that year. That winter, "Baseball America" rated him the tenth-best prospect in the Rays' minor league system.