The Cherries have paid an initial £9.4m for the 22-year-old Spaniard, with a further £1.7m available in add-ons.
Juanlu arrives with more than 100 senior appearances for Sevilla, having broken into the first team as an 18-year-old, and brings experience of the Champions League, Uefa Super Cup and LaLiga. He is also an Olympic gold medallist, having scored the winning goal in Spain's semi-final victory over Morocco at the Paris 2024 Games before beating France in the final.
"I am very happy, both for me and for my family," Juanlu told the club, adding that he had long wanted to make the move.
Bournemouth's president of football operations, Tiago Pinto, praised the defender's level of experience "at just 22," pointing to his record in Europe and LaLiga as evidence he could compete consistently at the top level.
The signing comes as Bournemouth confirmed defender Julian Araujo has undergone surgery on a left thigh injury sustained during the club's pre-season training camp in Austria, ruling him out for several months.
The Mexico international was forced off during Bournemouth's opening pre-season friendly against St Pauli, with further assessment determining an operation was required.
Araujo, 24, joined Bournemouth from Barcelona in 2024 but has struggled to establish himself at the Vitality Stadium, making 14 appearances in all competitions. He spent the second half of last season on loan at Celtic, where he impressed before that spell was also cut short by a quad injury in March. His latest setback is likely to delay any decisions over his long-term future at the club until his rehabilitation is complete.
Bournemouth said Araujo would be "supported by the club's medical department" through his recovery.
Juanlu becomes Bournemouth's third summer signing after the arrivals of Antonio Silva from Benfica and Alvaro Rodriguez from Elche, and is expected to be first-choice at right-back under head coach Marco Rose, with the more experienced Adam Smith moving into a backup role.
He is set to sign a long-term contract with the Cherries, who identified him as a target after sending Alex Jimenez out on loan to Fiorentina earlier in the window.