Van Persie, who is being deployed as a lone front man, has failed to meet expectations having netted just once while Dutch playmaker Wesley Sneijder is joint-top scorer at the finals with Spain's David Villa, both having struck five goals.
But for Van Marwijk there is no reason to doubt Van Persie's qualities as his form has got better during the tournament.
"Robin has really improved during the last few matches and I still have a lot of confidence in him," he said on Thursday. "I think he will show his best football in the coming match."
Van Marwijk has a full squad to choose from for Sunday's final at Soccer City with all his players fit and Gregory van der Wiel and Nigel de Jong, who were both suspended for the semi-final against Uruguay, also available again.
The coach revealed that after they went 3-1 up in Tuesday's match he began thinking they were close to reaching their third World Cup final, three decades after the 1974 and 1978 finals.
"I'm not the type who thinks about his own presence in such a match, I just approach it as the most important match in my life," he told Thursday's news conference.
"We (the technical staff) talk a lot about it but we're not overreacting. Actually, we handle it like we always did although we notice through all the media attention we're talking about a World Cup final but that does not change our preparation."
"We've won six in a row this campaign and also had some phases of good football but sometimes we were too sloppy... when we didn't reward a good fluent attack with a goal," he added.
