When Ryosei Akazawa, Japan’s chief trade negotiator, departed Tokyo on Monday morning for the latest round of tariff discussions in Washington, he left behind a nation in political turmoil and bracing for a new jolt of economic pain.

Mr. Akazawa’s party, the Liberal Democrats, had suffered a crushing defeat in elections the previous day, rendering it a minority governing party in both houses of Parliament.

Japan was also barreling toward an Aug. 1 deadline to reach a trade agreement with the United States. The Trump administration had vowed to impose a punishing 25 percent blanket tariff on Japanese goods shipped to the United States — the largest buyer of its exports.

The successful negotiation of a deal was widely seen as critical to the political survival of Shigeru Ishiba, Japan’s prime minister and a longtime ally of Mr. Akazawa. But after more than three months and seven rounds of tariff negotiations, few trade experts were expecting a deal to come to fruition.

Turns out the eighth time was the charm.

In a social media post late in the U.S. evening on Tuesday, President Trump declared that he had reached a “massive” trade deal with Japan. Japan had agreed to open its markets to more imports of American cars and rice, as well as invest $550 billion in the United States, he said.

In return, the president said that Japanese exports to the United States would be charged a tariff of 15 percent, lower than the 25 percent he was previously threatening. Critically, Japanese officials later said, car and auto part exports would be subject to a 15 percent tariff, lower than a separate and damaging 25 percent tariff that Mr. Trump had already imposed on the sector.