Instead of saying it will shut down three vehicle assembly plants, GM said the plants "will be unallocated in 2019".
GM's actions add up to the biggest restructuring for the U.S. No. 1 carmaker since its bankruptcy a decade ago, and mark a turning point for the North American auto industry.
GM shares were trading around $37.78 as of 3:35 ET on Monday.
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"This industry is changing very rapidly", Barra said during a press briefing.
"The UAW and our members will confront this decision by GM through every legal, contractual and collective bargaining avenue open to our membership", the union said in a statement.
The move provoked an immediate rebuke from President Donald Trump, who warned GM it was "messing with the wrong person" - and demanded the auto maker shut factories in China instead.
The General Motors Hamtramck Assembly plant in MI and Lordstown Assembly plant in OH are being "unallocated" in 2019 as a part of a major company restructuring. Trump says GM needs to "get a vehicle that is selling well and put it back" into the OH plant.More news: Astha Sarmah, ‘Average’ High School Graduate, Schools Donald Trump On Climate Change
GM said it will close three North American auto assembly plants next year: the Oshawa plant in Ontario, Canada; Hamtramck in Detroit, Michigan and Lordstown in Warren, Ohio. The totals are imprecise in part because there may be some double-counting with employees who are taking the October buyout offer.
Barra is reducing staff from operations that make conventional cars and hiring people who can design electric cars or add software prowess to GM Cruise, the automaker's autonomous vehicle unit in San Francisco. Most of the affected factories build cars that won't be sold in the US after next year.
Further plants in White Marsh, Maryland, and Warren, Michigan will be "unallocated". For instance, in July 2017 during a rally in Youngstown, Ohio, near the the president said of manufacturing jobs that state lost over the years: "They're all coming back".
GM's stock price surged on the heels of the announcement, closing at 37.65, up almost 5 percent on the day.
At the factories, around 3,300 blue-collar workers could lose jobs in the U.S. and another 2,600 in Canada, but some United States workers could transfer to truck or SUV factories that are increasing production. GM says the move is part of a transformation for the future, which includes an increased focus on next generation battery-electric vehicles.
The transition will come at the expense of about 8,000 salaried workers, and 6,000 hourly workers will either lose their jobs or be reassigned to other plants.
"They say the Chevy Cruze is not selling well", Mr. Trump told reporters, recounting his conversation with Ms. Barra.
When you factor in effects on the supply chain, some 49,000 jobs overall in North America could be affected.
"Every time we have a problem with a point, I just put up a picture of a Chevrolet Impala", the USA president was reported to have said. USA sales of the Impala were down 13 percent through September.