The Football Association confirmed on Monday that it will take no action in regard to the allegation.
Footage emerged which appeared to show Fellaini spitting near to Zabaleta's head while the City defender lay on the ground following a clash between the pair in United's 3-0 home derby defeat last Tuesday night.
It is understood referee Michael Oliver did not mention the incident in his match report, but the FA could have opted to retrospectively punish Fellaini if it had felt the midfielder had been guilty of misconduct.
George Boyd was banned for three matches after he spat at Manchester City goalkeeper Joe Hart earlier this month.
Oliver booked Fellaini moments before the alleged spitting incident for an elbow on the Argentina full-back.
United manager David Moyes said last week he had discussed the allegation with Fellaini and said the Belgium international was adamant he was in the clear.
"I've spoke with Marouane and he says it isn't the case," Moyes said after the 3-0 defeat.
"We will work with the FA if that is what we have to do. Marouane is adamant that at no time did he spit at Zabaleta.
"I don't think it is completely conclusive, from what I've seen."