Fellowes is teaming up with composers George Stiles and Anthony Drewe for the new West End project.
He says, "The Wind in the Willows is one of our greatest classics and as true and entertaining now as it has ever been. I am delighted and tremendously flattered to have been asked to write the book (narrative) for the new musical.
"In fact, I suspect this is something I've been wanting to do subconsciously for many years."
It is not yet known when or where the musical will debut.