Lev I Danylovych (baptismal name – Onophrius; between 1225 and 1229 – ca 1301) – second son of King Danylo Romanovych of Halych and Anna, daughter of Prince Mstyslav Mstyslavych the Bold, statesman, Prince of Peremyshl (ca 1240–1269), Belz (after 1245–1269), and Halych (1264–1301).
Second son of King Danylo Romanovych of Halych and Anna, daughter of Prince Mstyslav Mstyslavych the Bold. In 1247 (according to other information, in 1251–1252) Lev Danylovych married Constance, daughter of Hungarian King Béla IV, to strengthen the union with the Hungarian Kingdom. From a young age took part in military actions of his father. In 1252 helped him to fight against the Mongol-Tatar hordes headed by Voivode Kuremsa. After the death of his father (1264) Lev inherited the Peremyshl Principality and Halych land with the newly-founded city of Lviv (named after him), and after the death of his brother Shvarn Danylovych (ca 1269) got Kholm, Belz, and Dorohochyn Principalities and the town of Halych. In 1272 he transferred the capital of the Halych-Volyn state to Lviv. Lev I Danylovych spent the long years of his reign in Halych and Volyn lands in almost ceaseless wars with neighbours: Poland, Hungary, Lithuania, and the Yatvingians, notrejecting unions with Polish and Lithuanian princes to geteven with his brothers, first of all Volodymyr Vasylkovych, Prince of Volodymyr-Volyn.
After the death of Krakow Duke Boleslaw V the Chaste (1279) Lev Danylovych in alliance with Bohemian King Václav II Přemyslid attempted to capture Krakow. Supporting in the struggle for the Krakow throne Boleslaw of Mazovia (son of his sister Peredslava), waged a protracted war against Polish Duke Leszek the Black. He annexed part of Transcarpathia with the towns of Mukacheve and Berehove (Berehove land; ca 1280) and the Lublin land (ca 1292) to the Halych-Volyn Principality.
In his old age Lev Danylovych became a monk. During the reign of Lev II Danylovych the seal with the historical emblem of Halych-Volyn Rus – ‘the Rus lion’ was first used.