Bournemouth aren't wasting any time.
Kelly frequently played at left-back for the Robins over the 2018-19 campaign, but he's also a strong option at center-half. He made his league debut for Bristol City in December 2017 and first appeared for the England Under-21s in November of last year.
"Lloyd is an exciting talent and a player of real potential, while at the same time already possessing good experience for someone so young," Bournemouth manager Eddie Howe said of his new arrival.
"He is athletic, versatile, physically excellent and has a very bright future ahead of him."
Kelly will challenge Charlie Daniels for the left-back slot at Dean Court. Fellow full-back Diego Rico, who was signed from Leganes last summer, has struggled to establish himself as a regular starter in Howe's squad.
The young Englishman's versatility should also allow him to appear at center-back in four- and three-man defenses. Over the final months of this past season, Nathan Ake, Steve Cook, and Chris Mepham often occupied those positions, with the latter moving to Bournemouth from Brentford in January.
The Cherries finished the latest Premier League season in 14th place, 11 points above the final relegation spot.