The Real Madrid star is said to have been stopped at high sea after leaving a restaurant on the paradise island of Formentera near Ibiza. Officials are believed to have boarded the yacht on Tuesday afternoon.
Ronaldo, 32, was reportedly on the yacht with girlfriend Georgina Rodriguez and friends and family when the officials approached the vessel on a large boat.
The routine inspection is said to have lasted around an hour and a half.
Spanish society magazine Hola, which broke the news on Wednesday, published pictures on its website of three officials boarding the yacht.
Hola reported, “A Customs camera filmed everything that happened during the inspection.
“One of Cristiano’s relatives was the person who dealt with the administrative work and showed them the documentation they asked for.
“Ronaldo stayed very calm and at no time did he get up to speak to the officials or ask about what was happening.
“He was with his girlfriend Georgina and his mother Dolores Aveiro, who he was seen chatting with inside the yacht.”
The footballer and his entourage were understood to be on their way back to Ibiza when they were stopped.
The inspection is believed to have formed part of routine checks performed on vessels in the area, related to the paperwork on board to make sure the correct taxes have been paid.
The Customs agents are understood to have left after discovering everything on board Cristiano’s yacht, named as AYa London, was in order.
Spanish Tax Agency officials are not expected to make any official comment.
A source quoted by leading Spanish daily El Mundo described the inspection as “a routine control on vessels of the type that are carried out every summer in the Balearic Islands”.
Customs agents are understood to have made their unexpected appearance just off the Formentera shoreline as Cristiano returned from a restaurant with his mother, girlfriend and children including his new surrogate twins Eva and Mateo.