The show, about a friendly postman who delivers parcels in a quaint English village, was a huge hit with children in the 1980s, and film executives are working on a movie adaptation to mark its 30th anniversary.
And Harry Potter star Grint, who grew up watching the series, can't wait to get to work on the animated project.
He says, "I used to watch Postman Pat when I was a small boy. My brother and sisters all did at some stage.
"Postman Pat is one of those British symbols, I suppose. You are totally familiar with him and his cat Jess, even if you have never watched the series."
Episodes actor Stephen Mangan will voice Pat.