The crowd was less than half the 67,619 who attended the Selecao's previous appearance at FedEx Field, against the U.S. in 2012.
Jair Marrufo, one of the American referees at this year's World Cup, gave Neymar the yellow card in the 44th after the forward attempted to dribble past defender Bryan Tamacas.
Richarlison got his first two international goals, and Phillipe Coutinho also scored for Brazil.
Neymar converted a penalty kick in the fourth minute after Richarlison was taken down. With 59 goals in 92 international appearances, he is third in Brazilian history behind Pele (77) and Ronaldo (62).
Neymar dribbled past two defenders in the 16th minute before laying off a pass to Richarlison, who one-timed a shot from the edge of a penalty area in his second international appearance.
Neymar hit the crossbar in the 22nd minute, Coutinho scored from Neymar's squaring pass in the 30th minute and Richarlison got his second goal in the 50th when he pounced on a loose ball. Neymar's corner kick in the 90th minute was headed in by Marquinhos.