Barrichello has reached a deal to race for KV Racing Technology and will make his debut at the season opener on March 25 in St Petersburg, Florida.
The 39-year-old Brazilian will participate in all 16 IndyCar races on the calendar, including the five on oval tracks. He previously had said there was a chance he would skip the oval circuits because of safety concerns and in consideration of his family.
Barrichello drove for Williams in F1 last season but lost his spot to countryman Bruno Senna, ending a 19-year career in the series.
KV Racing co-owners Kevin Kalkhoven and Jimmy Vasser were also at Thursday's news conference to announce the deal, as were Barrichello's new team-mate and old friend, Tony Kanaan. EJ Viso of Venezuela will be the team's other driver this season.
There had been widespread rumors of Barrichello moving to IndyCar since he tested twice for KV Racing earlier this year on an invitation from Kanaan. He drove well and had said his move to the series depended only on finding sponsors.
The arrival of a well-recognised name in racing such as Barrichello is welcomed by IndyCar, which is trying to revamp its image after ending last season on a low following the death of Dan Wheldon in Las Vegas.
Izod IndyCar Series CEO Randy Bernard went to one of Barrichello's test sessions to personally meet the Brazilian.
Barrichello drove in 325 races in F1 - a record in the series - finishing second in the drivers' standings in 2002 and 2004, both times behind seven-time champion Michael Schumacher of Ferrari.