"I'm very much looking forward to making my IndyCar Series debut this season and am proud to be racing with a team of such high calibre and pedigree as that of Andretti Autosport,” Rossi, 24, said in a statement on his team's website.
"As a racer through and through, I cannot wait to get started; our goal is to be competitive immediately at the first race in St. Petersburg."
The Californian will take over the seat left vacant when Bryan Herta Autosport merged with Andretti last week to form a four-car team that includes former Indy 500 winner Ryan Hunter-Reay and Marco Andretti, son of team owner Michael Andretti and grandson of American racing great Mario Andretti.
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Rossi also becomes part of the team that features the last three American-born drivers to race in Formula One, along with owner Michael Andretti and Andretti's Global Rallycross driver Scott Speed.
Rossi, who last year also finished runner-up in GP2, the top developmental series for F1, has been clear that his ultimate goal remains to race Formula One and become the first American to win a grand prix since Mario Andretti in 1978.
"We're really excited to have Alexander join the team," said Michael Andretti. "His credentials speak for his ability and we're confident he'll transition seamlessly into Indy car racing.
"It's neat to have the last three American F1 drivers in 25 years to be under one roof. I think it speaks to the level of talent we have."
(Reporting by Steve Keating in Toronto; Editing by Frank Pingue)
