Knoyle enjoyed an outstanding 2020-21 in United's promotion-winning campaign, with his performances earning recognition from his peers by way of a place in the League Two Team of the Year.
London-born Knoyle, 24, was an ever-present for the U’s, starting each of United’s 46 league games last season and was described as the best right-back in League Two by head coach Mark Bonner, who was keen to keep the player.
But with Knoyle being out of contract at Cambridge at the end of June, he has elected to move on under freedom of contract and join Rovers, which sees him link up again with Rovers manager Richie Wellens, who the former West Ham academy product previously worked with at Swindon Town.
The former England under-18 player, who had loan spells earlier in his career at Dundee United and West Ham, has signed a two-year deal.
Knoyle arrival follows on from last week's addition of Portsmouth midfielder Ben Close.