The Cardinals said the I-70 rivals will finish their season series with a split doubleheader Thursday. The Royals won the series opener in St. Louis 10-7 on Tuesday. The Cardinals took two of three in Kansas City last month.
Royals top prospect Jac Caglianone made his major league debut in the series opener, going hitless in five at-bats.
The forecast called for rain to continue well past the scheduled start time Wednesday.
St. Louis right-hander Miles Mikolas had been set to face Royals lefty Noah Cameron before the postponement. The probable starters for the early game Thursday weren't immediately announced.
