Embraer says in a statement that the agreement includes options for an additional 47 aircraft, which would bring the total value of the deal to approximately US$4 billion.
Deliveries of the 76-seat aircraft are expected to begin in mid-2013, Embraer said.
The new aircraft will be operated by the Indianapolis, Indiana-based company under the American Eagle brand in American Airlines' regional network. The deal is subject to court approval expected in the first quarter of 2013.
With the new order, the Republic Airways E-Jet fleet will be comprised of 72 E170s and 107 E175s, the Embraer statement said, adding that more than 150 E175s are currently in service with 12 carriers.
Embraer is the world's third-largest commercial aircraft manufacturer behind Boeing and Airbus.
On Wednesday, American Airlines said it reached a 12-year deal with Republic Airways to operate regional flights using 76-seat planes.
Under the agreement, Republic Airlines will get Embraer planes painted in American Eagle colors. They will go into operation at a rate of two or three per month starting in mid-2013.