Rossi will team up with Christian Rasmussen, who steps up to a full-time IndyCar program next year.
Rossi and Rasmussen will drive the #20 and #21 entries respectively while owner-driver Ed Carpenter will return for the Indianapolis 500.
The team confirmed its two full-timers have been signed on multi-year contracts.
Rasmussen was pivotal in the team’s quest to make the leaders circle, which pays roughly $1.5 million (AUD) in prize money.
“It is an exciting day to not only welcome Alex to the team but to extend our relationship with Christian,” said Carpenter.
“We are very excited about our lineup, and other partnership announcements that will be forthcoming.
“As excited as I am about these additions, I also want to thank Rinus (VeeKay) for his five years with the team.
“It is always hard to say goodbye to a teammate and a friend, but I am confident that Rinus’ career will continue to blossom.
“As we look ahead to 2025 and beyond, I am fully focused on getting ECR back to new heights, winning races and contending for championships.”
Next year will mark the 10th season for Rossi in the IndyCar Series. He drove for Andretti Global from 2016 to 2022 before a two-year spell at McLaren.
Rossi will be replaced at McLaren by former Rahal Letterman Lanigan Racing driver Christian Lundgaard of Denmark.
“I’m honoured that Ed (Carpenter) and the entire ECR ownership group recognise the value I can bring to the team,” said Rossi.
“Their commitment to excellence, along with the power of Chevrolet and the ambitious plans they have for the future made this opportunity impossible to pass up.
“I’m eager to get started and contribute to the team’s continued success.”
Rasmussen, who won the 2023 Indy NXT series, scored a season-best ninth at Mid-Ohio in his first year of IndyCar.
“I’m incredibly excited to earn a full-time role with Ed Carpenter Racing,” said Rasmussen.
“My rookie year in IndyCar was full of firsts, and I can’t wait to bring everything I’ve learned into my second season.
“I’m extremely grateful for this opportunity and looking forward to team up with Alexander (Rossi) to push the team forward and fight towards the front of the grid more consistently.”