AP Photo/Richard Drew
Former Mexican President Vicente Fox in New York in 2007.
"We don't like him. We don't want him. We reject his message," Fox said in a CNN interview.
He added: "I don't understand why President Peña has offered this opportunity. I think it's nothing more than a political stunt. Trump is using Mexico, is using President Peña to boost his sinking poll numbers."
Fox, who once said it was "very important" to invite Trump to Mexico, said that Peña Nieto was making a "big mistake" inviting the real-estate magnate. He noted Trump's pledge to deport approximately 11 million immigrants living in the US illegally and force Mexico to fund a physical wall along America's southern border.
"I think President Peña is taking an enormous political risk by hosting Trump," Fox said. "If he's perceived as going soft on Trump, it will hurt him greatly - he will even be considered like a traitor."
Fox said Peña Nieto needs to to force Trump to apologize for "stupid proposals" like starting a trade war with Mexico and the deportation force.
Asked about the Trump campaign's recent waffling on key tenants of his immigration pitch to Republican primary voters, Fox said he still didn't believe Trump.
"You cannot trust a guy like this. How can we grant confidence and support to ideas that change every day?" Fox said.
He added: "He's still firm on his stand that he will throw them out, that he will build the wall. He can't play both sides."
Trump immediately responded to Wednesday's interview, noting on Twitter that Fox invited him to visit Mexico earlier this year.
https://twitter.com/mims/statuses/770971306766381056
Former President Vicente Fox, who is railing against my visit to Mexico today, also invited me when he apologized for using the "f bomb."
Neither Fox nor Trump have appeared particularly friendly toward one another in the past.
Fox challenged Trump to a debate earlier this year in Mexico and said that he was "not going to pay for the f------ wall."
Trump responded in kind, demanding the former president apologize for his profanity-laced comments.
'This guy used a filthy, disgusting word on television, and he should be ashamed of himself and he should apologize, okay?" Trump told an audience at a campaign rally in February.