A judge in Spain has reopened a sexual-assault case against Alwaleed bin Talal, a nephew of the Saudi king and one of the world’s richest men. Bin Talal, the largest individual stakeholder in Citigroup and the second-largest investor in News Corp., is accused of drugging and raping a 20-year-old model on a luxury yacht in the Mediterranean in 2008. Medical tests at the time found traces of semen and a sleep-inducing chemical in her urine. However, a judge in Ibiza, noting there were no signs of physical violence, closed the case in 2010 for lack of evidence—until the appeals court reopened it Tuesday, ordering a lower court to continue investigating and summon bin Talal to appear. Bin Talal says he learned of the accusation for the first time Tuesday and denies any wrongdoing, claiming he was never in Ibiza in 2008.