Yamamoto has already picked up early professional experience in Japan, making his senior debut for Gamba Osaka in February and going on to make six appearances as a creative, attack-minded number eight.
He is regarded as one of the most promising talents of his age group and is also a Japan Under-19 international. The 18-year-old, who stands at 1.77m, will initially link up with Dortmund's second team in the Regionalliga, though with the squad missing several senior players away on World Cup duty, he is expected to train with the first team during pre-season and could feature on the club's Asia tour, which runs from 26 July to 3 August.
Academy director Paul Schaffran explained the thinking behind the move, framing it as a considered piece of long-term recruitment rather than a short-term punt.
"Takato is a player who convinced us from the very start. He brings exactly the qualities we want to see in our game. We are convinced that he will take the next step in his development with us," Schaffran said.
Yamamoto himself struck an excited tone on completing the move.
"I still can't quite believe it. Borussia Dortmund — such a big club. It's the first time I've left Japan, everything here is new to me. But I'm ready. I want to learn and get better every day. And I will give everything to repay this trust," he said.
The move continues Dortmund's track record of scouting internationally for promising young talent, with Yamamoto now set to begin life in Germany during a summer pre-season programme shaped by the club's heavy involvement in the World Cup, before working out where he best fits into the club's development pathway over the course of the coming season.
Should he impress during his time in the Regionalliga and in and around the first-team environment, Dortmund retain the option to convert the deal into a permanent transfer, giving both club and player a clear route to assess whether the switch to European football proves the right fit long-term.