The move brings an end to a turbulent two years in west London for the 20-year-old, who was training away from Chelsea's first-team squad this summer and had not even been handed a squad number for the new season under head coach Xabi Alonso, a clear sign the club expected him to leave. Chelsea have retained a sell-on clause as part of the agreement.
Guiu's time at Stamford Bridge never recovered momentum after a loan to Sunderland collapsed almost as soon as it began. He joined the newly-promoted club last season only to be recalled just 25 days later, following an injury to their first-choice striker, but he was unable to force his way into Chelsea's plans on his return, making only three further appearances.
In total he featured 29 times for the Blues, contributing 10 goal involvements, six of which came during Chelsea's Conference League-winning campaign in 2024-25 — the season that represented the high point of his time at the club.
RB Leipzig sporting director Marcel Schäfer said Guiu's profile fit the club's identity.
"Marc's intensity, his high tempo and his incredible work rate suit our RB DNA very well," he said. "He brings enormous energy out of possession, but he's also a presence in the box with a clear instinct for goal. Despite being only 20, he has already played at the highest level and in the Champions League.
"We're convinced he can help us immediately with his quality and mentality, while still having huge development potential."
Guiu himself struck an optimistic tone on completing the move.
"Joining RB Leipzig is exactly the right step for me," he said. "The club made a real effort to bring me here and, in our talks, gave me a clear idea of the role I can play in the team. RB Leipzig have shown time and again that young players can develop here at the very highest level. They're also an ambitious club with high aspirations in every competition."
The Spain Under-21 international joined Chelsea from boyhood club Barcelona in July 2024 for a fee in the region of £5m, and arrives at Leipzig to address a pressing need up front, with regular striker Rômulo currently sidelined by a knee injury.
RB Leipzig are also working on the other side of the ledger this deadline day, with defender El Chadaille Bitshiabu closing in on a season-long loan move of his own to French side Lens, one that will reportedly include no option to buy.