Right-hander Jordan Zimmermann makes his first career start game series.
Cueto (9-5) gave up one run -- a ninth inning homer to Ryan Zimmerman -- while scattering six hits and fanning five.
The Dominican entered the game with a 1.94 earned run average, but was 1 2/3 innings shy of the minimum amount to qualify for the National League's ERA lead. After the win, his ERA was trimmed to a league-best 1.89.
Zimmerman had two hits and Rick Ankiel added a pair of singles for the Nats, who have lost four times in their last six games.
Ross Detwiler (1-3) was pegged with the loss after giving up a run on seven hits in six innings.
A third-year big-leaguer, Zimmerman has made 46 starts before facing the Reds, including a 4-3 loss to the Chicago Cubs at Wrigley Field in his last outing, where he gave up four runs on nine hits in 6 2/3 innings.
His last win came six days earlier in Denver, where he surrendered two runs on four hits in 5 2/3 innings of a 5-3 defeat of the Colorado Rockies.
The 25-year-old Wisconsin native is 3-5 in 10 home assignments this season and tossed seven scoreless innings in his last Washington appearance on July 31 while getting a no-decision against the New York Mets.
The Nationals won that game, 3-2, and are 6-4 in his last 10 starts.
For Cincinnati, veteran right-hander Bronson Arroyo tries yet again for his first win since June 25.
The Key West native won a 10-5 interleague decision at Baltimore to improve to 7-6 on the season, but has since gone 0-3 in eight starts while his earned run average has swelled from 5.01 to a more disturbing 5.31.
The Reds are 3-5 in those eight starts, including a 5-3 win over San Diego on Aug. 12 in which Arroyo got a no-decision for allowing an earned run six hits in six innings.
He is 3-2 lifetime against the Nationals in eight meetings with a 4.10 ERA across 48 1/3 innings.
Arroyo debuted with the Pittsburgh Pirates in June 2000, when Zimmermann was 14 years old.