The victory was the ninth for the 23-year-old Busch in NASCAR's second-tier series. He has won eight Cup races and three more in the Craftsman Truck Series.
The busy Busch's 70th race of the season followed a familiar script for Joe Gibbs Racing, whose teams have won 18 of the 31 Nationwide races.
Busch started 16th, but his superior No. 18 Toyota was on display early. He quickly moved to the front and led 137 of the 200 laps.
Busch pulled away from Jeff Burton on a restart with three laps to go, despite taking just two tires on his final pit stop. Burton held on to finish second and Brian Vickers was third.
Points leader Clint Bowyer finished fourth and Carl Edwards was fifth.
The race, which included 14 Sprint Cup drivers and six in the Chase for the championship, never got into a flow thanks to rain and wrecks.
There were two rain delays totaling more than an hour, and 13 cautions that caused 58 laps to be run under caution.
But a finish well after midnight didn't affect Busch, who completed a Nationwide sweep at Lowe's Motor Speedway, and softened some of his disappointment from his recent weeks in the Sprint Cup.
Busch hasn't finished better than 15th since the season-ending Chase began, dropping him to 11th in the standings behind leader Jimmie Johnson.
But in a field that included several Cup veterans and inexperienced drivers, nobody could challenge Busch.
Johnson, the Sprint Cup points leader with five victories here in NASCAR's top series, remained winless in Nationwide races at the track. Fighting a loose car, Johnson hit the wall with 83 laps left, ending his chances.
The storyline of the soggy race week here - the running feud between Edwards and Kevin Harvick
- produced no further fodder. A day after they were involved in an argument in the garage and had to be separated by their crew members, the two played nice on the track despite running side-by-side much of the night.