Sometime Indy Lights race winner Jamin, announced by United last week, has also been guaranteed a spot in the team's line-up for this year's Le Mans 24 Hours.
United team boss Richard Dean said of Aberdein: "We have filled our last LMP2 seat with a great driver, who I know will get to grips with LMP2 quickly.
"Both Phil, who has so much experience now with prototypes and Tom, who knows the ELMS well, will be on hand to help Jonathan settle into the team.
"I’m confident that the three of them will get on brilliantly and they will all be fighting at the front come the opening round in Barcelona.”
Jonathan Aberdein, BMW Team RBM, BMW M4 DTM
Photo by: Daniel Reinhard
Aberdein progressed as far up the single-seater ladder as European Formula 3 in 2018, the year before switching over to the DTM with Audi customer squad WRT.
After finishing 10th overall in 2019, he was tipped for a possible promotion to Audi's works team before being snatched away by BMW, but the 22-year-old could do no better than 11th in the standings as the Bavarian marque struggled badly.
His best result in 2020 was a fourth place at Zolder, which matched the placing he achieved twice with WRT the previous season.
Aberdein commented: “I’m thrilled to be joining UnitedAutosports, together with Tom Gamble and Phil Hanson, this year for the European Le Mans Series.
"UnitedAutosports had a very impressive year last year, proving themselves time and time again and ultimately bringing home the championship title. We will be looking to repeat that level of success again this year.
"I believe we have a very strong driver line up and combining that with the engineering talent at UnitedAutosports, I’m sure we will be a force to be reckoned with.”
Duqueine announces 2021 drivers
Elsewhere on the ELMS LMP2 grid, the Duqueine Team has announced a refreshed line-up for the 2021 season, with Tristan Gommendy joined in the team's Oreca by two new teammates in the form of Rene Binder and Memo Rojas.
Cool Racing has also announced that it will field an unchanged line-up of Nicolas Lapierre, Antonin Borga and Alexandre Coigny, withdrawing from the WEC to focus on ELMS. Lapierre will dovetail his duties with the team with his new Alpine drive in the WEC's top class.