Rakuten, which plunged to an operating loss in 2020, is under pressure on multiple fronts as it battles Amazon in e-commerce and takes on Japan's cash-rich telcos with its own mobile network.
"Rakuten is the best partner for us, as it has advanced digital technology," Japan Post Holdings Chief Executive Hiroya Masuda told a news conference.
Friday's deal makes Japan Post Holdings the biggest shareholder in Rakuten outside the founding Mikitani family, and is part of a 242-billion-yen ($2.2-billion) share sale to companies including Tencent and Walmart.
A former state-owned utility that was later privatised, Japan Post Holdings has a presence on almost all Japanese high streets, with its postal unit having around 24,000 post offices nationwide.
"As tech giants grow in power we're combining our strength," said Rakuten's chief executive, Hiroshi Mikitani.
This month, chat app operator Line merged with SoftBank's internet business Yahoo Japan in a deal first announced in 2019.
Shares of both Rakuten and Japan Post rose in afternoon trade in Tokyo after Reuters and other media reported the tie-up.
Image Frame Investment, a unit of Chinese tech giant Tencent, will take 3.65% in Rakuten through third-party allotment, with Walmart also to buy shares.
Last year Rakuten acquired a 20% stake in Walmart's Japanese supermarket unit. It has had a partnership with the unit, Seiyu, since 2018.
(Reporting by Takashi Umekawa, Sam Nussey, Tim Kelly and Noriyuki Hirata; Additional reporting by Chang-Ran Kim; Editing by Christopher Cushing and Clarence Fernandez)
By Takashi Umekawa and Sam Nussey