Marianne Loir or Marie-Anne Loir was a French painter who specialized in portraits.

Marianne Loir was born in Paris around 1715, daughter of the goldsmith Alexis II Loir and granddaughter of Nicolas Loir. Her brother, Alexis III Loir (1712–1785), was a renowned sculptor.

She studied under Jean François de Troy (1679–1752), director of the French Academy in Rome, where Marianne stayed between 1738 and 1746. She became a member of the Académie des Beaux-Arts in Marseille in 1762, and seems to have stayed at Pau for a time in the 1720s and Toulouse. In 1763 she was in Paris, where she completed a portrait of the young Antoine Duplas on 1 September. She left Paris in 1765 and moved to Provence. She died sometime in 1769.

She left ten paintings signed and dated between 1745 and 1769.