Jerry Garcia's Captain Trips hat is valued at £30,000-50,000 ($47,070-78,450) ahead of a sale at Christie's London.
Garcia wore the iconic hat regularly during concerts and photo shoots during the mid 1960s, after picking it up in a fancy dress shop.
He later gave it to a friend, Harry Tsvi Strauch, who ran a boutique in the Haight-Ashbury in San Francisco - popular with hippies.
Strauch told Christie's: "In the Summer of 1966, The Grateful Dead became our neighbors, moving into 710 Ashbury, the house two doors up from ours.
"September 1967, an anti-War/Peace march was to come through Haight Street for a rally at Kezar Stadium. My wife - Hyla Strauch - was designing a Red, White & Blue American Flag themed window display for our shop in support of the march.
"She thought that Jerry Garcia's Capt'n Trips hat would be the ideal, iconic centerpiece for the display. We brought this up to Jerry, and he gave us the hat to use in the shop window!
"When it was time to take the display down, we offered to return the hat - Jerry Told us 'I don't want the hat...You keep it, take care of it.'"
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