The Welsh midfielder's goal in the 62nd minute proved enough for the Gunners to register a 1-0 win at Signal Iduna Park in Group F.
"He just shows how difficult it is to have assessment on players because one year ago everyone said 'yes, but Ramsey cannot score goals. He's in box in the right positions, but he can't score'," Wenger told reporters. "This season he scores game after game.
"I was always positive with him because when you watched the games again he was always in situations where he could be dangerous.
"This season he has gained confidence and he has an unbelievable engine to go from box to box. Confidence and his ability, of course, makes him a dangerous player."
Arsenal are currently level with Napoli on nine points at the top of the group, while Dortmund are third with six.