"The new system is not against Russia. I want to categorically stress this, Romania [will] not host a system against Russia, but against other threats," he said.
Russia is adamant that nuclear arms should be deployed only in the territory of the states possessing such weapons.
"Russia's official position is that only that missile defence is good which is missile defence together with Russia"
Pavel Felgenhauer, military analyst
US tactical nuclear arms should be withdrawn from Europe, Andrei Nesterenko, the Russian foreign ministry spokesman, said on Thursday.
"Issues of further nuclear disarmament, including tactical nuclear arms, should not be addressed as such, but only in close relation with other types of weapons, including conventional armed forces in Europe and the ballistic missile defence systems.
"In this context, withdrawal of American tactical weapons from Europe back to the United States would be welcome. It should be accompanied by complete and irreversible demolition of the entire infrastructures supporting the deployment of such weapons in Europe," he said.
The announcement of a planned missile defence in Romania comes months after Washington shelved a plan to place missile defence facilities in the Czech Republic and Poland - a move welcomed by Moscow.
Pavel Felgenhauer, a military analyst, said Russia had feared that missiles placed in Poland could be used to attack Moscow with nuclear warheads.
"That was a bit far-fetched but that was believed in Moscow, he told Al Jazeera from the Russian capital.
"The missiles that apparently will, maybe, some day, be put into Romania will be smaller missiles and Romania is further from Moscow than Poland is.
"Russia's official position is that only that missile defence is good which is missile defence together with Russia, a joint control.
"But at least this will be seen as not as dangerous, most likely, as one with Poland, so I don't think that right now there will be that much fuss about it."
The US embassy in Bucharest said that Washington had "determined that Romania is well-suited for the location of this system to provide protection for European Nato allies".
A plan unveiled last September by Barack Obama, the US president, includes land-and sea-based missile systems in and around the Gulf to defend against what it says is a growing Iranian missile threat.
The US offer was brought to Bucharest by Ellen Taucher, under-secretary of state for arms control who leads a team of American experts in Romania, Basescu said.
Teodor Baconschi, Romania's foreign minister, said the plan was first presented to Basescu during a visit by Joe Biden, the US vice-president, to Bucharest in October but was not made public.
"This became official today," Baconschi said.
In past years, Romania's parliament has solidly backed participation in US and Nato-led military ventures, including Romanian troop deployments to Iraq and Afghanistan.