Alexei Nikolaevich (Russian: Алексе́й Никола́евич) (12 August [O.S. 30 July] 1904 – 17 July 1918) was the last Tsesarevich (heir apparent to the throne of the Russian Empire). He was the youngest child and only son of Emperor Nicholas II and Empress Alexandra Feodorovna. He was born with haemophilia, which his parents tried treating with the methods of a peasant faith healer named Grigori Rasputin.

After the February Revolution of 1917, the Romanovs were sent into internal exile in Tobolsk, Siberia. After the October Revolution, the family was initially to be tried in a court of law, before the intensification of the Russian Civil War made execution increasingly favorable in the eyes of the Soviet government. With White Army soldiers rapidly approaching, the Ural Regional Soviet ordered the murder of Alexei, the rest of his family, and four remaining retainers on 17 July 1918. Rumors persisted for decades that Alexei had escaped his execution, with multiple impostors claiming his identity. Alexei's remains, along with those of his sister Maria (or Anastasia), were ultimately discovered in a secondary grave near the rest of the Romanov family in 2007. On 17 July 1998, the 80th anniversary of their execution, Alexei's parents, three of his sisters, and the four retainers, were formally interred in the Cathedral of St. Peter and Paul, while Alexei's and Maria's (or Anastasia's) bones remain in Russian state archives. The Romanov family was canonized as passion bearers by the Russian Orthodox Church in 2000.

Alexei is sometimes known to Russian legitimists as Alexei II, as they do not recognize the abdication of his father in favor of his uncle Grand Duke Michael as lawful.