Hassan Pirnia (Persian: حسن پیرنیا ‎; 1871 – 20 November 1935), titled Moshir-od-Dowleh, was a prominent Iranian politician of 20th-century Iran.

He held a total of twenty-four posts during his political career, serving four times as Prime Minister of Iran.

Upon his father's death, he assumed the title of Moshir al Dowleh, playing an important role in drafting the Persian Constitution of 1906.

From 1907 to 1908, Pirnia was the Minister of Foreign Affairs, during which time he declared the Anglo-Russian Entente, which would divide Iran into zones of imperial influence, as null and void.

One of Pirnia's key actions during his time as Prime Minister saw him help prevent the introduction of the Anglo-Persian Agreement of 1919.