Rossiter, who was initially signed as a test driver, has been promoted to the race seat for the 6 Hours of Monza following original signing Magnussen's surprise return to Formula 1 with the Haas team.
SUPER GT race winner Rossiter made his WEC debut in a Lotus LMP2 car in 2012 before moving to the top class with ByKolles, making sporadic appearances with the LMP1 outfit between 2016-19.
Rossiter will be joined by the team's other five contracted drivers Jean-Eric Vergne, Paul di Resta, Loic Duval, Gustavo Menezes and Mikkel Jensen, although the exact make-up of the two cars is yet to be revealed.
However, Rossiter is so far only signed up for the Monza race and it remains unclear who will take his place in the team in the following two rounds of the season at Fuji and Bahrain.
The announcement was made at a launch event at Portimao where Peugeot also revealed the race-ready version of the 9X8 Le Mans Hypercar in its definitive grey-and-black livery, complete with green accents.
The colour scheme is an evolution of the design that was seen at several public appearances and test outings since an earlier iteration of the car was first revealed last year.
As expected, Peugeot has decided against adding a conventional rear wing in the final race-ready version of the 9X8, with much of the downforce instead generated from the underbody of the car - an area where the teams are offered considerably more freedom than in the previous-generation LMP1 rules cycle.
Peugeot also confirmed that it will join the championship from the fourth round of the season in Italy on July 10, having previously only stated that it will begin its campaign "this summer" after the Le Mans 24 Hours.
This means that the French manufacturer will only contest the second half of the six-round series, taking part in the Fuji and Bahrain events following its debut in 6 Hours of Monza in a little under two month's time.
Peugeot last won Le Mans outright in 2009 with the diesel-powered 908 HDi FAP and continued racing in top-level prototype racing for two more years, before the board abruptly decided to axe its LMP1 programme prior to the rebirth of the WEC in 2012.
When Peugeot finally returns to the Circuit de la Sarthe in 2023, having elected against making an early return to the French endurance classic this year, it will face competition from Toyota, Ferrari, Glickenhaus, Porsche and Cadillac among others.
