Adams, 56, said he was traveling through Cairo International Airport on Tuesday ahead of an outdoor concert at the Pyramids, reported NBC News.
After initially "refusing to release the equipment" with "zero" explanation, customs officials finally returned the instruments, which had been scrawled on with green indelible marker, according to the singer.
This was likely "some sort of customs clearance identification," Adams added.
He took to Instagram to show the marking on 1957 Martin D-18 guitar, worth thousands of dollars.
The instruments "were all marked - even my harmonicas! So silly," Adams said.
A source at the airport said that the incident was "nothing new, the system is the same."
The official, who spoke on condition of anonymity because he was not authorised to speak to the media, said, "We mark it to be sure the same item that came in the country left the country."
This wasn't the first time Adams' instruments have been tagged by airport officials. He took to Twitter in July 2015 to allege that one of his guitars had been written on by Air Canada staff.
He was keen to stress that the incident had "not dampened my love for Egypt, I love the country and its people" but said that was "just extremely unfortunate that the customs people at the airport had no respect for our musical instruments.
