Jani, 39, will share the French squad's Oreca 07-Gibson with Rene Binder and silver-rated teenager Nico Pino.
It follows the news in December that Jani would be ending a 10-year stint as a Porsche works driver after missing out on a seat in the German manufacturer's new LMDh project for 2023.
The Swiss driver, who also tested for the Meyer Shank Racing IMSA team last October, said he would evaluate his options to stay involved in prototype racing in the wake of his exit from Porsche.
He said this could involve seeking a drive with one of the teams running the new Porsche 963 on a customer basis in either the FIA World Endurance Championship or IMSA SportsCar Championship, or even a switch to LMP2.
Jani has never done a full season in LMP2, but did drive the Oreca 07 for Rebellion Racing in 2017 in the Rolex 24 at Daytona and Sebring 12 Hours.
Duqueine is going into the new ELMS season with an entirely new driver line-up after fielding a car for Richard Bradley, Memo Rojas and a revolving cast of third drivers in 2022.
Bradley and Rojas finished 14th in the standings with best finishes of sixth at Imola (alongside Reshad de Gerus) and Barcelona (alongside Anders Fjordbach).
Binder previously drove for Duqueine in 2021 but sat out last season, while 18-year-old Pino moves up from the LMP3 class, in which he finished second overall last year for Inter Europol Competition.
Duqueine has also said that it will submit an entry for the Le Mans 24 Hours with its Oreca LMP2 car.
Last week ex-F1 driver and current Peugeot sportscar runner Paul di Resta announced an ELMS campaign for 2023, with Toyota ace Jose Maria Lopez also set to race in the series this season.
TF Sport has also gained Louis Deletraz from Prema for his title defence in ELMS for 2023.
