Elvis' historic 1942 Martin D-18 guitar -- which The King owned between 1954 and 1956 -- will be up for auction over at Gotta Have Rock and Roll, and no doubt it'll go to a serious collector with huuuuge pockets. The minimum bid starts at a whopping $1.2 million. It's expected to fetch up to THREE MILLION BUCKS.

The investment's totally worth it ... because the instrument oozes musical history. For starters, the guitar is often referred to as Elvis' "Sun Sessions" guitar. Elvis recorded at the iconic Sun Studios in Memphis, TN and also performed with the guitar onstage.

The King of Rock 'n' Roll bought the guitar at O.K. Houck's Piano Store in Memphis, trading in his Martin 000-18 model. He then went on to decorate the body of the guitar with adhesive metal letters spelling out "ELVIS." Today, the guitar's missing the "S." The extensive wear visible on the guitar, no doubt, is due to Elvis' hard strumming and the guitar's frequent use.

And, get this ... during the time Elvis owned the guitar, he also recorded the classics "That's Alright (Mama)," "Blue Moon of Kentucky" and "Good Rockin' Tonight," to name a few.

The new owner of this guitar will get TONS of paperwork proving the authenticity ... from a photocopy of the receipt from O.K. Houck Piano and photocopies of correspondence from the curator of the Country Music Hall of Fame to a historical note about the guitar ... which has been reported to be Elvis' first guitar. The auction opens on July 22. Good luck!!!