False Dmitry III, also called Pseudo-Demetrius III (Russian: Лжедимитрий III), was the last and most enigmatic of three pretenders to the Russian throne who claimed to be the youngest son of Ivan the Terrible, tsarevich Dmitry.

Supposed to have been a deacon called Sidorka, he appeared suddenly, from behind the river Narova, in the Ingrian town of Ivangorod, proclaiming himself the tsarevich Dmitry Ivanovich, on the 28th of March 1611. The Cossacks, ravaging the environs of Moscow, acknowledged him as tsar on the 2nd of March 1612, and under threat of vengeance in case of non-compliance, the gentry of Pskov also kissed the cross to the thief of Pskov, as he was usually nicknamed. On the 18th of May 1612 he fled from Pskov, was seized and delivered up to the authorities at Moscow, and there executed.

See also

• Time of Troubles
• Dymitriads
• False Dmitry I
• False Dmitry II