The German publication confirms stories from Japan that Union have eyes on the Yokohama F. Marinos player.
Endo, 22, is a product of the Yokohama youth system and has become a regular in the J.League.
He can play on both flanks but is primarily a left winger.
Capped twice by Japan late last year, Endo has a current market value of €1.3m.
Union Berlin have had two players from Japan on their books in the past.
In the 2005-06 season, Yuzuru Okuyama made seven league appearances for the club.
And more recently, full-back Atsuto Uchida played two games for Union in the second half of the 2017-18 campaign.
Endo has officially joined Union on a season-long loan deal with an option to buy.
— 1. FC Union Berlin (@fcunion) July 25, 2020