The 34-year-old Saracens captain is due to take to the pitch for the last time on Saturday when his side faces Northampton Saints in the Aviva Premiership final.
Borthwick, who has won 57 caps for England, will work under former Saracens director of rugby Eddie Jones as Japan's forwards coach following his retirement.
"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."
Borthwick started his career with Bath in 1998 before joining Saracens in 2008.