The Nationals selected Rodney's contract from Triple-A Fresno and selected Venters' contract from Double-A Harrisburg after signing them to minor league deals over the past month.
Rodney, 42, joined the Nationals organization on June 1 after being designated for assignment by the Oakland Athletics.
The right-hander is a three-time All-Star but posted a 9.42 ERA in 17 appearances this season with Oakland.
Venters, 34, had a 17.36 ERA in nine appearances this season with the Atlanta Braves.
The left-hander also is a former All-Star and was named the National League's Comeback Player of the Year in 2018, when he posted a 3.54 ERA for Atlanta after returning from his third Tommy John surgery.
The Nationals enter play Tuesday with a 6.29 bullpen ERA, the worst mark in the major leagues.