The Standard Missile-3 Block IIA is being built by Raytheon and Mitsubishi Heavy Industries as part of the U.S. Missile Defense Agency's sea-based Aegis Ballistic Missile Defense System.
Aerojet said the contract from Raytheon Missile Systems runs through 2016.
A planned production program would follow.
"This is an important win for Aerojet," said Vice President of Missile Defense and Strategic Systems Michael Bright. "It is the culmination of more than 18 years of company-sponsored research and development and government program work at Aerojet.
"We look forward to working with Raytheon on completing the development of this very important missile defense capability for use in the defense of the United States and Japan."
The Block 1 SMi missile is deployed on U.S. Aegis cruisers and destroyers and Japanese destroyers as a defense against short- to-intermediate-range ballistic missile threats.
The missile intercepts an incoming missile in the ascent and mid-course phases of flight.
The missile under development will feature larger rocket motors and a larger kinetic warhead.