"I could have remained in power if I had surrendered the sovereignty of Saint Martin Island and allowed America to hold sway over the Bay of Bengal," the letter obtained by The Economic Times read.
Hasina said last summer that Bangladesh would not lease the island, without specifying the details or the interested party. US State Department spokesperson Matthew Miller said days later that the US had never mentioned plans to take control of the island.
Hasina said she chose to step down "so that I did not have to see the procession of dead bodies" and urged the South Asian nation not to be manipulated by radicals.
"If I had remained in the country, more lives would have been lost, more resources would have been destroyed. I made the extremely difficult decision to exit," she wrote.
Deadly protests erupted across Bangladesh after the announcement of a multi-day "non-cooperation action" with the authorities by the Anti-Discrimination Student Movement on Sunday. Clashes between students, police, and government supporters escalated into riots that reportedly left more than 440 people dead. Hasina resigned amid the unrest.