The Lac-Mégantic rail disaster occurred in the town of Lac-Mégantic, Quebec, Canada, on 6 July 2013, at approximately 01:15 EDT, when an unattended 73-car Montreal, Maine and Atlantic Railway (MMA) freight train carrying Bakken Formation crude oil rolled down a 1.2% grade from Nantes and derailed downtown, resulting in the fire and explosion of multiple tank cars. Forty-seven people were killed. More than thirty buildings in Lac-Mégantic's town centre, roughly half of the downtown area, were destroyed, and all but three of the thirty-nine remaining buildings had to be demolished due to petroleum contamination of the townsite. Initial newspaper reports described a 1 km (0.6-mile) blast radius.

The Safety Board identified multiple causes for the accident, principally leaving a train unattended on a main line, failure to set enough handbrakes, and lack of a backup safety mechanism.

The death toll of 47 makes this the fourth-deadliest rail accident in Canadian history, and the deadliest involving a non-passenger train. It is also the deadliest rail accident since Canada's confederation in 1867. The last Canadian rail accident to have a higher death toll was the St-Hilaire train disaster in 1864, which killed 99.