Agreement is believed to have been sorted with Gamba Osaka before the weekend and the midfielder will head for Elland Road when the transfer window opens next month.
He will be loaned abroad immediately because Football Association rules will prohibit him from gaining a work permit for the rest of the English season.
He has not made the requisite number of international appearances for Japan to meet the FA’s requirements.
However, Leeds view Ideguchi effectively as a summer 2018 signing and are confident he will secure a work permit for the 2018-19 campaign.
The J-League campaign was completed last month and United see a European loan as a way of keeping Ideguchi active.
The next Japanese season will commence in the Spring.
Leeds were known to be tracking Ideguchi towards the end of the summer transfer window with a view to signing him.
He had come to attention during Japan’s successful qualification campaign for the 2018 World Cup, scoring in a 2-0 win over Australia that guaranteed the country’s involvement in Russia.
He also scored in stoppage-time as Japan played North Korea over the weekend.
Speaking to Japanese media, Ideguchi indicated that he was ready to take up a move to Elland Road.
“I think this is a place where I can learn and improve as a player,” he said.
“There are opinions that it is better to wait six months (until after the World Cup), but I want to go there without being influenced by people’s opinions.”
The deal between Leeds and Osaka is believed to be worth around £500,000 including bonuses.