The unpublished letters from Nicolas I, Alexander II and their relatives fetched 743,000 francs ($835,000) — far above the estimate of 60,000-80,000 francs.
According to auction house Hôtel des Ventes (HDV), most were purchased by the same buyer at Monday night's sale.
The letters, written in Russian, French and English, were sold by an American who had received them from the widow of a US captain deployed in Europe during World War II.
The soldier, who was also a journalist, came across the historical treasures in 1945 as he was covering the end of the war for US publication Stars and Stripes.
Most were addressed to Grand Duchess Olga Nicolaievna, the daughter of Nicolas I and sister of Alexander II, who moved to Stuttgart after marrying a German prince — later to become King Charles I of Wurttemberg.
Through the missives, "intimate secrets and plots of the Russian Court are unveiled as well as the beliefs and political pains of the two emperors," HDV said.
The auction house meanwhile said it had, upon request from the Geneva judiciary, withdrawn from sale an unpublished set of photographs of Alexander III's family.
Russia's consulate had urged Geneva prosecutors to halt the sale of the pictures and help clarify their origin.
HDV says the snaps were rescued by Otto Hofmann, a German Bauhaus artist who was serving with the Nazi forces when they destroyed the Gatchina palace in 1944.
"Acting against martial law and putting his own life at risk, the soldier hid and brought back around 30 images which he considered historical evidence and culturally important for Imperial Russia," the auction house explained.
The family of Hofmann, who managed to hide the shots during several years as a prisoner of war, had decided to sell the pictures to "reveal his gesture to all," HDV said.
The auction house's annual Russian Imperial Sale, which in addition to the letters included a range of valuable objects from the period, pulled in a total of 3.8 million francs, or nearly four times its pre-sale estimate of one million francs.
A rare pair of porcelain vases made in 1849 and given by Nicolas I as a gift to his sister-in-law Elena Pavlona were the highlight of the sale.
Estimated at between 300,000 and 500,000 francs, they finally went for 1.58 million francs.