These and other reports said calls for the 68-year-old to depart had grown louder within the LDP since the results of the Upper House election.
But Ishiba communicated his decision after striking a trade deal with Washington that cut a threatened 25% tariff to 15% before an August 1 deadline.
In the election on Sunday, the LDP and its junior partner Komeito fell three seats short of retaining a majority.
It came only months after Ishiba’s coalition was forced into a minority Government in the more powerful Lower House, in the LDP’s worst result in 15 years.
Ishiba won the party leadership in September, on his fifth try, to become the 10th LDP Prime Minister since 2000 – all of them men.
Since the October snap Lower House election, the ruling coalition has been forced to bargain with Opposition parties to pass legislation.
After years of stagnant or falling prices, consumers in the world’s fourth-largest economy have been squeezed by inflation since Russia’s 2022 invasion of Ukraine.
In particular, the price of rice has doubled, while resentment has also lingered over an LDP funding scandal.
“I really hope things will get better in Japan, but the population is declining, and I think living in Japan will get tougher and tougher,” Naomi Omura, an 80-year-old from Hiroshima, told AFP in Tokyo on Wednesday.
“It is disappointing that Japan cannot act more strongly” towards the United States" but “I think it was good that they agreed on a lower tariff”, she said.
Tetsuo Momiyama, an 81-year-old Tokyo resident, said Ishiba “is finished already”.
“It’s a good timing for him to go,” Momiyama said.
– Agence France-Presse