Chacin (6-3) limited the Nationals to five hits while walking just one and striking out three. The right-hander has posted a sparkling 1.74 ERA over the span of three straight wins.
Chacin supplied an RBI single during a three-run fourth inning that put the Rockies ahead 6-0.
DJ LeMahieu went 3-for-4 with a homer and two RBI and Nolan Arenado also went deep for Colorado, which rebounded after losses in the first two games of the series.
Nationals starter Dan Haren (4-9) was lit up for six runs on seven hits over just 3 1/3 innings.
Ryan Zimmerman homered for Washington.