MADRID — A Spanish judge has reopened an abandoned sexual assault case against a Saudi prince who is one of the world’s richest men, reviving accusations that he raped a 20-year-old model on a luxury yacht in the Spanish Mediterranean in August 2008.

The prince, Alwaleed bin Talal bin Abdulaziz al-Saud, a nephew of King Abdullah of Saudi Arabia, is the largest individual stakeholder in Citigroup and, among his other major holdings, is the second largest investor in the News Corporation. Forbes valued his fortune this year at $19.4 billion, making him the 26th richest man in the world and the single richest in the Arab world.

The accuser did not go public, and the original complaint appears to have remained largely unknown. The case was quietly closed in July 2010 for what a judge on the Mediterranean resort island of Ibiza called a lack of evidence. But on appeal, a Spanish provincial court for the Balearic Islands, which has jurisdiction over Ibiza, ordered the judge to resume investigating and to summon the prince to appear.

The provincial court said the judge, Carmen Martín Montero, was on vacation and could not be reached for comment.