"This will help us to deal with the new threats of the 21st century, of long-range missile threats from countries like Iran or from North Korea," US Secretary of State Condoleezza Rice said before she signed the accord with Poland's Foreign Minister Radoslaw Sikorski.
The signing comes amid heightened tensions between the United States and NATO, and Russia over the conflict with pro-Western Georgia.
But Rice again sought to fend off criticism.
"It is defensive and is not aimed at anyone," she said.
"It is nonetheless a system that establishes firmly again, and reaffirms, our cooperation and relationship with Poland. It will deepen our defence cooperation and it will deepen our ability to deal with threats."
Polish Prime Minister Donald Tusk said the deal "achieved the basic aims that mean Poland and the United States are more secure".
Washington plans to base 10 interceptor missiles in Poland plus a radar facility in the neighbouring Czech Republic between 2011 and 2013.
Both hosts are NATO members since 1999 and the missile shield will complete a system already in place in the United States, Greenland and Britain.
Russia has rejected the US argument that the shield, which was endorsed by all 26 NATO member states earlier this year, is meant to fend off potential missile attacks by what Washington calls "rogue states".
Moscow claims the timing of the deal is further proof the system is aimed at Russia -- a suggestion rejected by Washington.
Moscow had already dubbed the shield a security threat designed to undermine Russia's nuclear deterrent.
"We will be forced to respond to this adequately. The EU and US have been warned," Russian President Dmitry Medvedev said last month as the missile talks moved to a close.
Last week, Russia's General Anatoly Nogovitsyn said Poland was making itself a target "100 percent".
Polish President Lech Kaczynski hit back in a televised address Tuesday, saying his country would not give in to threats.
"No one can dictate to Poland what it should do. That's in the past," Kaczynski said.
"Our neighbours should now understand that our nation will never give in, nor allow itself to be intimidated," he added.
Kaczynski did not name Russia directly, but his mention of the "past" was a clear reference to Poland's post-World War II decades as a Soviet satellite.
"No one should be afraid of (the shield), if they have good intentions towards us or the rest of the West," Kaczynski said.
Warsaw and Prague have had rocky relations with Moscow since they broke free from the Soviet bloc in 1989, and ties have worsened since they joined NATO in 1999 and the European Union in 2004.
To try to calm Moscow's ire, Poland has repeatedly offered to allow Russian inspections of the missile facilities.
US and Polish negotiators signed a preliminary deal in Warsaw last Thursday, capping 15 months of negotiations.
Talks had ground on until the United States accepted Poland's demands for extra security guarantees to offset the potential risks of hosting a base -- not specifically from Russia -- including a Patriot missile air-defence system and boosted military ties.
The missile plan foresees the deployment of several hundred US troops in Poland to service the shield facility as well as the Patriot missiles, which will gradually be turned over to the Poles once they have been trained to use them.
Washington and Prague sealed the radar deal in July.
Both accords must still be ratified by Polish and Czech parliaments.
© 2008 Agence France Presse