Michael Bay says the first time he heard Peter Cullen speak as Optimus Prime, chills shot down his spine … and decades later, losing him left Bay in tears.

The “Transformers” producer/director sat down with TMZ’s Charlie Neff and recalled learning about Cullen’s death last week … saying he let the news sink in for about an hour before he cried.

Michael has since put together a tribute video honoring Cullen … looking back at the man and the voice that defined Optimus Prime.

He says he’ll never forget hearing Cullen record Optimus for the first time … stunned that such a booming voice was coming from a relatively small, cowboy boot-wearing horse guy.

Michael’s now helping make sure Cullen’s legacy lives on outside the movies … telling us he’s working with the Make-A-Wish Foundation in Peter’s honor, at the request of Cullen’s family.

Bay says he’s hosted plenty of Make-A-Wish kids over the years and called the organization amazing … adding that’s how Peter’s loved ones want him remembered.

As TMZ reported, Cullen diedWednesday at his Los Angeles home at 85, passing peacefully surrounded by loved ones.

Bay says billions of people have heard Cullen’s voice over the years … and, to him, Peter embodied everything Optimus Prime was supposed to be.