The 38-year-old toured twice with the Lions in the noughties, travelling to Australia and then New Zealand on successive tours.
Grewcock played in all three Tests against the Wallabies in 2001 before featuring in the first international against the All Blacks four years later.
He made a total of 11 Lions appearances, scoring a solitary try on his debut against Western Australia a decade ago.
Grewcock has enjoyed a professional career that has spanned 15 years and featured successful stints at Saracens and Bath after making his name with Coventry.
A World Cup winner in 2003, and subsequently an MBE, Grewcock has been at Bath for the past 10 years and has made 226 appearances for a club he intends to remain with in a mentoring capacity.
"I have been incredibly lucky to play for such a long time at such a great Club," said Grewcock, who will be focusing on bringing young players through the schools system and into the Bath Rugby Academy.
"I have enjoyed every minute of it, and have been very fortunate to have been given the opportunity to stay involved throughout. I'm really looking forward to working with the young players at the Club, and helping them to progress with their careers."
Having made his England debut in 1997, Grewcock went on to win 69 caps for his country, winning a Grand Slam in 2003 but missing the 2007 World Cup through suspension.
"Danny is one of the most respected players in the game today, and will remain so even after his retirement," said Lions and Bath boss Sir Ian McGeechan.
"He has reached the highest level a player can, and should be very proud of his lengthy career.
"His hard work, dedication, and commitment to the game, both on and off the field, serve as a great example to any young player, and I look forward to working with him at Bath in his new capacity."