PARIS — To the Spanish authorities, Picasso’s 1906 “Head of a Young Woman” is a unique painting, the only example in Spain from a pivotal period in the artist’s life. Because of that status, the courts labeled it a “national treasure” that should not be sold outside the country.

In the view of Jaime Botín, a member of a wealthy Spanish banking dynasty, the work is simply his personal property. Purchased in 1977 and kept on a yacht docked along Spain’s Mediterranean coast, it is valued at as much as 26 million euros, or $28.3 million, in today’s booming art market.

But on Friday, after an apparent tip from the Spanish authorities, it was seized by French customs officials from the yacht, which had docked in Corsica. The Spanish government contends that Mr. Botín was trying to move the Picasso to Switzerland for sale, in defiance of a court ruling invoking a Spanish law meant to shield such works of art from export.

It is only the latest collision of powerful forces in the art world, where demand from the superrich has enticed owners to sell their treasures, even as national governments scramble to keep such works at home.