The prototype fretless guitar, made in the 1960s, appeared in Sunday's show.
Its current owner recorded sessions for a film company co-founded by Harrison, who asked him to play the guitar.
"I played a few notes and he said: 'Yeah, you're definitely getting more out of it than I am. It's doing better for you, why don't you have it'."
Bartell's of California made the instrument, which the owner, Ray, described as "a strange old thing to play".
A photograph in the episode shows the guitar among Harrison's collection.
Antiques Roadshow expert Jon Baddeley said: "What's really important is the provenance and there it was in the photograph - in George Harrison's collection. Wow, you can't do better than that."
Valuing the guitar at between £300,000 and £400,000, he added: "I think in 25 years it's by far the most expensive thing I've ever seen."
During the episode, filmed at Battle Abbey in Sussex, Mr Baddeley said: "To a guitar collector, it's initially a very rare guitar.