TOKYO — Toyota said on Friday that it was buying a 5 percent stake in Mazda, its fellow Japanese automaker, and that the companies would jointly build a new assembly plant in the United States.

The factory’s location has not been decided, but Toyota and Mazda said they hoped the first vehicles would roll off its production lines in 2021. The plant is likely to cost $1.6 billion and will employ about 4,000 workers, they said.

Akio Toyoda, Toyota’s chief executive, said in January that the carmaker would invest $10 billion in the United States over the next five years.

Although plans for that investment predated the election of President Trump, the announcement in January was widely seen as a response to Mr. Trump’s vows to promote American manufacturing — and to push back against countries like Japan that have large trade surpluses with the United States.

The alliance between Toyota and Mazda represents a small but significant step in the consolidation of the Japanese car industry, where a half-dozen different producers compete for customers and capital.

In an era of soaring development costs and unsettling technological shifts — especially the emergence of battery-powered and self-driving vehicles — many smaller producers fear they lack the resources to keep up.

Japan’s smaller carmakers, including Mazda, Suzuki and Mitsubishi, have sought partnerships with larger producers before. Mazda and Suzuki were for decades partly owned by Ford and General Motors, respectively, but their American partners withdrew under financial pressure after the 2008 financial crash.

Mitsubishi last year joined the Renault-Nissan alliance, after the French-Japanese group extended Mitsubishi a $2.2 billion lifeline to help it recover from a scandal over falsified fuel-economy ratings.

Toyota has also been extending its reach.

Last year it took over its longtime minicar affiliate, Daihatsu. It has been strengthening its links with Fuji Heavy Industries, the maker of Subaru cars, in which Toyota owns a 16.5 percent stake. And it has been discussing a potential new partnership with Suzuki.

Toyota and Mazda have been cooperating since 2010, when Toyota agreed to license its gasoline-electric hybrid drive system to Mazda. The companies expanded the scope of their agreement in 2015 to include development of other environmentally friendly technologies.

With the Prius and other hybrids, Toyota has dominated the market for emission-saving vehicles for years. But as fully battery-powered cars gain favor with regulators and consumers, it faces new challenges — both from traditional competitors and new players like Tesla.

Mazda is known for making powerful and fuel-efficient internal combustion engines, but it lacks its own electric alternatives. Its sporty image and widely praised designs could appeal to Toyota: Mr. Toyoda has repeatedly spoken of his desire to give his company’s products more flair.