The Great New York City Fire of 1845 broke out on July 19, 1845, in Lower Manhattan, New York City. The fire started in a whale oil and candle manufacturing establishment and quickly spread to other wooden structures. It reached a warehouse on Broad Street where combustible saltpeter was stored and caused a massive explosion that spread the fire even farther.

The fire destroyed 345 buildings in the southern part of what is now the Financial District, resulting in property damage estimated at the time between $5 million and $10 million (equivalent to between $139 million and $278 million in 2020). Four firefighters and 26 civilians died. The Great New York City Fire of 1845 was the last of three particularly devastating fires that affected the heart of Manhattan, the other two occurring in 1776 and 1835.

While very destructive, the 1845 fire confirmed the value of building codes restricting wood-frame construction, which had been implemented in 1815. In spite of general improvements, the 1845 fire prompted public calls for a more proactive stance in fire prevention and firefighting, and the city established a reserve unit called the Exempt Fireman's Company.