The Japanese driver joins British duo Oliver Jarvis and Phil Hanson in the line-up in the #22 Oreca 07-Gibson, one of three LMP2 cars United will field in the ELMS this season, after spending the past three seasons in F2.
Sato made his F2 debut towards the end of the 2019 season with Campos Racing after winning that year's Euroformula Open title, and joined Trident for his first full season the following year.
After two campaigns that yielded a single point each, Sato switched to the front-running Virtuosi squad last year but failed to improve on his previous best finish of eighth, scoring six points for 22nd in the standings.
“I would like to thank everyone in the team for their trust and everyone around me for making this happen," said Sato, who also drove for the AlphaTauri Formula 1 team in the 2020 young driver test in Abu Dhabi.
"It will be a new challenge for me but I’m sure we can have a great season – I’m excited to get started."
Team co-owner Richard Dean added: “This is a super strong driver line-up. We’ve strengthened our team in so many areas as we have our sights solely focussed on winning back our European Le Mans Series title.”
Jarvis will also be contesting his first full season in the ELMS, which he will combine with his FIA World Endurance Championship commitments for United after parting ways with the Meyer Shank Racing IMSA WeatherTech SportsCar Championship team last winter.
"I know Phil extremely well and look forward to working with Marino for the first time having watched him in F2," said Jarvis.
"It will be my first full season in the championship but being part of a proven race-winning team, with extremely strong teammates, I’m confident we can challenge for victories.”
United had previously announced the line-ups for its other two LMP2 entries in the ELMS for 2023.
Ex-Formula 1 racer and current Peugeot WEC driver Paul di Resta is joined by Guy Smith and Jim McGuire in the #23 entry, while a third car will be shared by Nelson Piquet Jr, Andy Meyrick and Daniel Schneider.
