Glynn joins Harlequins
Twomey, who injured his foot early in the 2016/17 season, has had to undergo a further operation which will prolong his recovery beyond the start of the new campaign.
Glynn joined the Bristol Rugby Academy at the age of 18 and progressed into the senior team making a total of 140 appearances.
The former England Under-18 helped Bristol to promotion into the Premiership for the 2016/17 season and made 18 appearances.
"It is really disappointing to see that Sam Twomey has had to undergo a further operation on a foot injury he sustained last season," said Harlequins Director of Rugby John Kingston.
"Ben will provide excellent cover for the coming season; he is an experienced operator who has played many times in the Premiership.
"He will complement the locks we have within our senior roster."
Glynn added: "I am very excited about joining Harlequins for the coming season, it is a great opportunity for me.
"I'm looking forward to getting stuck into the pre-season campaign with my new team mates and hitting the ground running for when the season begins in September."
