The bird dropped a surprise from above, hitting the King's suit jacket and splattering others around him including members of the press.
"It's well it didn't land on my head," the King quipped, according to Irene Marting, who witnessed the incident and met the composed monarch shortly afterward.
"Being hit by a seagull, it's supposed to be good luck," Marting said.
"We're so thrilled he's come to Northern Ireland to see us and it's really nice to see him in Newcastle."
Charles was visiting the southeast coastal town on the second day of a three-day visit with Queen Camilla.
The Queen was spared the spectacle because she was making a separate visit to Royal Hillsborough, where she tried her hand at pouring a pint of Guinness at a pub.
Camilla joked she was "not quite the expert but my husband is" while pouring the pint.
The King seemed to foreshadow his fate earlier on Wednesday during a visit to a food pantry when he picked up a roll of toilet paper and said, "very important".
A radio station on Wednesday apologised for "any distress caused" after accidentally announcing the monarch had died.
Radio Caroline in Essex said in a post on social media the announcement had been made on Tuesday afternoon because of a "computer error" at its main studio.
On Wednesday afternoon, playback for Tuesday's broadcast between 1.58pm and 5pm was unavailable on the station's website.
Station manager Peter Moore wrote on Facebook: "Due to a computer error at our main studio, the death of a Monarch procedure, which all UK stations hold in readiness while hoping not to require, was accidentally activated on Tuesday afternoon (May 19), mistakenly announcing that HM the King had passed away.
"Radio Caroline then fell silent as would be required, which alerted us to restore programming and issue an on-air apology."
Established in 1964, Radio Caroline is a former pirate radio station based in Maldon, according to its website and social media.
"We apologise to HM the King and to our listeners for any distress caused," it said.
with PA