The Martin D-28 left-handed guitar, which McCartney has signed, went up for auction on Wednesday, reported Contactmusic.
Proceeds from the sale will benefit the Nature Conservancy's African Elephant Initiative.
Alongside his signature, the former Beatles star, a keen animal activist, wrote, "This guitar saves elephants."
"To have Sir Paul's support and working with Martin Guitar is a real honour. This calibre of collaboration will make a difference to help end the worst poaching crisis in history," said David Banks, the managing director of the The Nature Conservancy's Africa Program.