Alexey, Alexei, Alexie, Aleksei, or Aleksey (Russian: Алексе́й [ɐlʲɪkˈsʲej]; Bulgarian: Алексей [ɐlɛkˈsɛj]) is a Russian and Bulgarian male first name deriving from the Greek Aléxios (Αλέξιος), meaning "Defender", and thus of the same origin as the Latin Alexius.

Alexey may also be romanized as Aleksei, Aleksey, Alexej, Aleksej, etc. It has been commonly westernized as Alexis.

Similar Ukrainian and Belarusian names are romanized as Oleksii (Олексій) and Aliaksiej (Аляксей), respectively.

The Russian Orthodox Church uses the Old Church Slavonic version, Alexiy (Алексiй, or Алексий in modern spelling), for its Saints and hierarchs (most notably, this is the form used for Patriarchs Alexius I and Alexius II).

The common hypocoristic is Alyosha (Алёша) or simply Lyosha (Лёша). These may be further transformed into Alyoshka, Alyoshenka, Lyoshka, Lyoha, Lyoshenka (Алёшка, Алёшенька, Лёшка, Лёха, Лёшенька, respectively), sometimes rendered as Alesha/Aleshenka in English. The form Alyosha may be used as a full first name in Bulgaria (Альоша) and Armenia.

In theory, Alexia is the female form. It is, however, almost non-existent in Russian-speaking countries. A popular Russian, Ukrainian, and Belarusian female name Olesia/Olesya/Alesia/Alesya is one possible form.

The patronymics based on Alexey are Alexeyevich (male) and Alexeyevna (female).

The following surnames derive from Alexey and its various forms:

• Alexeyev/Alekseyev
• Alexeyevsky
• Alyoshin
• Alyokhin (Alekhine, Alekhin)
• Lyoshin
• Alexeytsev/Alekseytsev /Alekseitsev
• Alexeychuk/Alekseychuk/Alekseichuk
• Alexeychik/Alekseychik/Alekseichik
• Alexeyuk/Alekseyuk
• Alexeyenko/Alekseyenko/Alekseenko
• Alexeychenko/Alekseychenko/Alekseichenko
• Alexievich

Alexeyevka is a common Russian village name.