The Indiana Jones star saddled up to join chat show host David Letterman on a ride through the streets of New York.
Incredulous passersby took photos as the guys trotted past on two horses named Chase and Shane.
Harrison was on Letterman's show to discuss his latest film Cowboys and Aliens, which also stars Daniel Craig.
They play enemy gunslingers who join forces to stop an alien invasion.
Initially, the veteran actor was sceptical about the quirky premise.
He said: "When I read it frankly I didn't get it."
"But I've wanted to do a western for 25 years, they just stopped making them.
"And it was clear that they'd found a new way to make them from the title."