The Manchester United forward was lying on the ground following a foul when Williams kicked the ball into the back of his head during Sunday's Premier League match at the Liberty Stadium.
United manager Sir Alex Ferguson said Van Persie was "lucky to be alive" after the incident in the 1-1 draw.
Martinez, who managed Swansea before joining the Latics in 2009, admitted he could understand Ferguson's reaction but does not believe Williams, a player he signed and knows well, acted out of malicious intent.
"You see the incident and it is a very, very dangerous situation," Martinez said.
"If you see one of your players with a bang on the back of the head it is really dangerous and it can have bad consequences. You can understand that.
"I signed Ashley Williams for Swansea and I know his character inside out. I can guarantee you without even looking at the incident that he is not a nasty footballer.
"He is a winner and fully committed, a leader and a captain, and I don't for one second doubt that he did not do that on purpose.
"It is an action of the game. He can show frustration - it is an emotional game and it is very unfortunate that it hits Van Persie in a part of the body where he could have been hurt heavily, and you don't want to see that.
"But from my point of view, I know Ashley Williams really well - when you sign a player you find out about a player and I can guarantee you he is not the type of footballer who would do that on purpose."