The Northeast Blackout of 1965 was a significant disruption in the supply of electricity on November 9, 1965 affecting Ontario, Canada and Connecticut, Massachusetts, New Hampshire, Rhode Island, Vermont, New York, and New Jersey in the United States. Around 25 million people and 80,000 square miles were left without electricity for up to twelve hours.

The cause of the failure originated at the Niagara generating station, Sir Adam Beck Station No. 2 in Ontario. At 5:16 PM Eastern time a single line of the power plant tripped, within seconds other lines out of the plant overloaded and also tripped, shutting down the plant generators. Within five minutes the power distribution system in the northeast was in chaos as the effects cascaded through the network breaking it up into 'islands', plant after plant experienced load imbalances and automatically shut down. The affected power areas were the Ontario Hydro System, St Lawrence-Oswego, Western New York and Eastern New York-New England. Maine, with only limited electrical connection southwards, was not affected.

Power resupply was uneven. For example, New York City was dark by 5:27. Parts of Brooklyn were re-powered by 11:00 and all of the borough by midnight but the entire city was not returned to normal power supply until almost 7:00 AM on the 10th.

Following the blackout measures were undertaken to try and prevent a repetition. Reliability councils were formed to establish standards and share information and improve coordination between electricity providers. Ten councils were created covering the four networks of the North American Interconnected Systems. The Northeast Reliability Council covered the area affected by the 1965 blackout.

The events of the blackout were dramatized in the 1968 film, Where Were You When the Lights Went Out?.

For information on other famous blackouts, see the List of power outages.

The myth of the blackout baby boom

A thriving urban legend arose in the wake of the Northeast blackout of 1965 in which it is told that a peak in the birth rate of the blackout areas was observed nine months after the incidence. The origin of the myth is a series of three articles published in August 1966 in the New York Times in which interviewed doctors told that they had noticed an increased number of births.

The story was debunked in 1970 by J. Richard Udry, a demographer from the University of North Carolina at Chapel Hill, who did a careful statistical study which found no increase in the birth rate of the affected areas.