A man has walked away uninjured after falling several hundred metres off a cliff in the Blue Mountains, west of Sydney.

Police say the 46-year-old was trail running on Mount Portal in the Blue Mountains National Park on Saturday afternoon when he slipped and fell hundreds of metres onto a small ledge.

He was unable to climb to safety because of the risk of falling further, police said.

Two men who were on a boat on the Nepean River below heard the man calling out for help two hours later.

The man was rescued from the site with the assistance of Polair, police said.