One source said Kasten wants to take time away from the game for personal reasons. However, another person in the industry believes a "lack of commitment from ownership" to spend on upgrading the roster was a factor in Kasten's decision.
Kasten, 58, is best known for his tenure as club president of the Atlanta Braves during their 1990s heyday. He had been president of the Nationals since 2006.
This season included highlights and heartbreak for the Nationals, due in large part to the debut and subsequent injury of star pitching prospect Stephen Strasburg. Washington is currently last in the National League East with a 64-88 record.
SI.com first reported that Kasten had stepped down.