The unidentified man was snapped with one hand on the instrument as he
navigated congestion on the 70mph A27 in West Sussex.
Witnesses and road safety campaigners slammed the suited driver as 'idiotic'.
He was spotted by coach passenger Nichola Carr, who was looking after a coach full of schoolchildren heading from Portsmouth to Crawley.
She said: "I was very shocked and stunned when I looked down and saw a man playing the guitar while behind the wheel.
"He was definitely playing the guitar and my first thought was: 'Oh my god, is he really doing that?'"
Neil Greig, director of policy and research at the Institute of Advanced Motorists, said: "You cannot drive and play the guitar at the same time.
"We always say you cannot do two things at once if one of them is driving - this is first time I have heard of someone playing guitar at the wheel."
Andrew Lord, owner of Brighton Guitars, added: "This is an absolute first for me. I have heard people doing other things while they drive - having sex, drinking, being on their phone, watching television - but never playing the guitar.