The 20-year-old is understood to be valued at around £25m, though the final structure of the fee has yet to be confirmed.
Guiu is nearing an exit from Stamford Bridge having joined from his boyhood club Barcelona in 2024 for around £5m, and he made 29 appearances in total for Chelsea, six of his eight goals coming during the club's Conference League-winning campaign in 2024-25.
The Spain Under-21 international spent a brief loan spell at Sunderland last season, making three appearances before being recalled, and has trained away from Chelsea's main first-team group throughout this summer amid growing expectations of a permanent departure.
Manager Xabi Alonso's crowded striking options, with Joao Pedro, Emanuel Emegha and Danny Welbeck all ahead of him in the pecking order, have left Guiu with limited pathways to regular first-team football in west London. He was not handed a squad number for the new season, a clear indication from the club that his future lay elsewhere, and he had also been linked with moves to Napoli and Roma earlier in the window before talks with Leipzig gathered pace in recent days.
RB Leipzig have a track record of developing young forwards into first-team regulars, and the move offers Guiu the platform to establish himself as an everyday starter in the Bundesliga. Chelsea are set to retain a sell-on clause as part of the agreement, ensuring a future financial return should the German club move him on again.
Separately, midfielder Kendry Paez has been recalled from an unsuccessful loan spell at Argentine side River Plate, freeing up one of Chelsea's overseas loan slots.
The Ecuador international managed limited playing time in South America and is now expected to join another club on loan before the window closes, potentially in England, though continental interest has also been reported. Paez, who joined Chelsea from Independiente del Valle after turning 18 last year, had previously spent the first half of the season on loan at sister club Strasbourg before being redirected to Argentina in January.