Hossein Pirnia, also known as Mo'tamen al Molk (Persian: حسین پیرنیا; 1875–1948) was an Iranian politician. His father, Mirza Nasrullah Khan, and his elder brother, Hassan Pirnia, both served as Prime Minister of Iran.
He served as speaker of the Parliament of Iran from 1914 to 1925, and again from 1928 to 1929.
Pirnia played a significant role in the drafting of the Persian Constitution of 1906 and served as Minister of Education in 1918 and Minister without portfolio in 1920. He was elected to every session of the Parliament (Majlis) from 1906, serving as speaker for more than eleven years in total. In 1943 he was elected from Tehran to the 14th session of Parliament but declined to serve.