The strike starting on Thursday threatens the health of South Korea's economy and will likely disrupt the global supply of semiconductors.
Under intense pressure from the government and business groups to avert a strike, the two sides had sought to hash out a deal on bonus payments before the 18-day strike begins.
Union leader Choi Seung-ho said the strike would go ahead as management had not come round on one remaining sticking point in talks mediated by the government.
"I want to make clear that we had accepted the final proposal presented by government mediator," he told reporters.
"We express deep regret and feel disappointed but the union plans to go ahead with the strike according to the law," Choi said.
Samsung Electronics said in a statement on Wednesday the union had insisted on "unacceptable demands" that included the size of bonuses for loss-making units.
"The reason an agreement could not be reached .... is that accepting the labour union's excessive demands would undermine the fundamental principles of company management," it said.
Its shares were down 3.1 per cent after the news.
The union had demanded that Samsung abolish a cap on bonuses that stands at 50 per cent of annual salaries, allocate 15 per cent of annual operating profit to bonuses and that these changes be formalised beyond one year.
South Korea's government threatened at the weekend to step in and order emergency arbitration, citing the adverse impact the strike could have on the economy.
The measure, which has been rarely employed, would prevent the strike from going ahead for 30 days while the government mediates talks.
Samsung accounts for almost a quarter of the country's exports.
It is also the world's largest memory chip maker and production disruptions could dent global supply at a time when the AI boom has caused shortages.
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