Nationals assistant GM Mike Rizzo announced the move on Wednesday.
The 26-year-old Kensing is 7-8 with a 5.21 ERA in 108 career games spanning five seasons with the Marlins. He's expected to be in uniform for the Nationals when they wrap up a series Wednesday night at Philadelphia.
To make room on the roster, Washington transferred outfielder Roger Barnadina to the 60-day disabled list.
The 24-year-old Gunderson was picked in the 48th round of the 2007 draft by the Nationals. He was 2-0 with a 2.84 ERA in five relief appearances for Class A Potomac this season.