Cambodian police on Sunday said they arrested one of the masterminds behind notorious file-sharing website The Pirate Bay, months after he was due to begin a one-year prison sentence in his native Sweden.

Gottfrid Svartholm Warg, 27 years old, was arrested Thursday in Phnom Penh in connection with the alleged illegal use of information technology, said police officials, adding that they were acting in concert with the Swedish government to get him out of Cambodia—and possibly back to Sweden to serve his term.

The...