U.S. Secretary of State Hilary Clinton attended the signing Saturday during a visit to the southern Polish city of Krakow.
With the agreement signed two years ago, the Bush administration had focused on protecting the U.S. against long-range intercontinental missiles with stationary interceptor rockets.
Since then, the U.S. has re-evaluated the threat from Iran, which Clinton says is developing short- and medium-range missiles capable of hitting Europe faster than previously thought.
Under the amended agreement, Poland will, starting in 2018, host a U.S. base, equipped with mobile interceptor rockets that can protect against missile attacks from any range.
Clinton expressed hope that Russia would drop its opposition to a U.S. missile defence system in Europe and accept an offer to co-operate in developing technologies for shooting down hostile weapons.
"The offer stands," she told a news conference after the signing, adding that the revised agreement would pose no threat to Russia.
Polish Foreign Minister Radek Sikorski said his country fully supports the amended plan.