Magnificent Sinner (original French title: Katia) is a 1959 French drama historical film by director Robert Siodmak about the romance between Tsar Alexander II of Russia and the then-schoolgirl Catherine Dolgorukova, who later became his mistress and finally his morganatic wife.

Produced in France, Magnificent Sinner stars Curd Jürgens as Tsar Alexander II, with Romy Schneider as the schoolgirl Katia who first becomes his mistress, before being elevated to the rank of princess.

The romance between a married emperor and an aristocrat leads to court intrigue and a weakening of the ties of loyalty between the Tsar's ministers and their ruler, and is instrumental in Alexander's ultimate assassination.

"[2] Bosley Crowther of The New York Times equally panned it as "an overstuffed costume picture" and "a hackneyed and ponderous bore."

"It would be just as good, if not better," he continued, "if it had been a tractor with which the Tsar fell in love.