The 21-year-old has signed a four-and-a-half-year deal at Elland Road that will see him tied down to the club until the summer of 2022.
Ideguchi made his international debut for Japan in June 2017 and has won 10 caps, scoring twice.
"Leeds United can today confirm the signing of Japanese International midfielder Yosuke Ideguchi," the Championship club wrote in a statement.
"The 21-year-old has successfully completed his medical and becomes the first Japanese player to sign for the club.
"Ideguchi joins the club from J1 League side Gamba Osaka for an undisclosed fee and has penned a contract at Elland Road until the summer of 2022."
Ideguchi has been loaned to Spanish second division side Cultural Leonesa for the rest of the season.