A Great Britain and England international, the stand-off made 406 appearances for Hull KR, scoring a club record 207 tries and kicking 607 goals.
Millward, a member of the Rugby League Hall of Fame, also won six trophies in 11 seasons as a KR coach.
Club captain Terry Campese will now wear the number 32 shirt.
Millward joined Hull KR from hometown club Castleford in 1966 and remained at the Robins until 1980, with his last appearance coming as captain in the 1980 Challenge Cup final win over rivals Hull FC - his fifth trophy as club skipper.
The family of Millward, who was awarded the MBE for his services to rugby league in 1983, will be presented a shirt on the KC Lightstream Stadium pitch when KR face Widnes on Sunday, 15 May.
A contribution from every ticket sold for that match will be donated to a charity of Millward's family's choosing.
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Hull Kingston Rovers have retired the number six shirt worn by club legend Roger Millward, who died at the age of 68 this week.