The lock-forward will become Japan's forwards coach, as the Cherry Blossoms prepare for next year's World Cup in England.
"Being a former captain of England, Steve has a good character and has great trust from the players," a Japan Rugby spokesman said.
"On the road to the World Cup, forwards will be the key to a successful campaign and we believe Steve as a forward coach will play a vital role," the spokesman added.
Japan on Sunday qualified for the 2015 rugby World Cup by thrashing Hong Kong 49-8 in Tokyo.
The victory handed them a seventh Asian Five Nations title.
Japan will have appeared in every World Cup to date and they will join South Africa, Samoa, Scotland and the United States in Pool B of the 20-team tournament.
The move sees Borthwick, who has helped the Japanese in the last two years in an ad-hoc capacity, formally link up with former Saracens director of rugby Eddie Jones. Former Australia coach Jones has been Japan's head coach since 2012.
Japan say they will look to Borthwick to hone their lineout skills.
The 34-year-old, who played 246 times for Bath before switching to Saracens in 2008, captained Saracens in Saturday's 23-6 Heineken Cup final loss to Toulon.
Borthwick will be hoping to go out on a high at the weekend, however, as Saracens face Northampton Saints in the English Premiership final at Twickenham on Saturday.
(Editing by Pritha Sarkar)
Share
