The 23-year-old will race in Singapore, Japan, the United States, Mexico and Brazil, with Merhi returning for the rounds in Russia and Abu Dhabi as Rossi completes his GP2 commitments.

Rossi becomes the first American to compete in F1 since Scott Speed’s final appearance for Toro Rosso in the 2007 European Grand Prix at the Nurburgring.

The Californian was reserve at the then-Marussia team for a time last season, taking part in practice at the Belgian GP, where he had briefly seemed set for a race drive in place of Max Chilton before it fell through.

“I’m very thankful to race for Manor and for their continued belief in me,” said Rossi.

“I’ve been prepared for this opportunity for quite a while now.

“Many will know from 2014 that the team and I already have a strong relationship and there’s a bit of unfinished business for me here.

“This is a small F1 team that has been through so much. They exemplify passion and true strength of character, and their comeback this season is extraordinary.

“I’m honoured to be part of this legacy and their continued growth and success.

“Singapore is a circuit I really enjoy, and the timing of my debut could not be more perfect.

“There’s a lot of work ahead and I’m looking forward to a busy end to the 2015 season.”

Team principal John Booth feels Rossi, overlooked for a seat at newcomer Haas next season as it prioritises experienced drivers, is “on the cusp of an exciting F1 career”.

Booth added: “Signing Alexander is a continuation of our proud record of providing young drivers with the opportunity to showcase their talents.

“We believe he will do a great job and look forward to seeing him race for the first time this weekend in Singapore.

“It is also fantastic news for the sport that it will have an American driver again, particularly with the United States Grand Prix on our horizon and now two races in South America, in Mexico and Brazil.

“I know Alexander is very excited about that and we look forward to providing him with the opportunity to show the American public what he can do.”

Merhi will remain present for Rossi’s five races as one of the team’s official reserves, alongside Fabio Leimer.

“While Roberto is obviously disappointed, he understands that this decision is in the long-term interests of the team and we thank him for his professionalism,” said Booth.

“No decision has been made regarding our 2016 driver line-up and we will continue to evaluate our options during the remainder of the season.”