Mattis's remarks come amid heightened tensions with Russia, which is preparing for massive military exercises.
Mattis's remarks come amid heightened tensions with Russia, which is preparing for massive military exercises in its western military region in September, and has deployed a missile system in neighbouring Kaliningrad.
"We will deploy whatever capability is necessary here," Mattis said when asked about regional air defences.
Two US defence officials earlier said the United States was considering sending a Patriot missile battery to the Baltic region for NATO air defence exercises this summer, though they stressed the move would only be temporary.
Moscow last year deployed nuclear-capable Iskander missiles into its Kaliningrad exclave, which borders Lithuania and Poland, rattling the NATO members.
Mattis blasted such moves.
"Any buildup of Russian combat power in an area where they know, and we all know, they are not threatened by anything that we are doing ... is simply destabilising," Mattis said at a press conference with Lithuanian President Dalia Grybauskaite.
He however refused to discuss whether Vilnius had requested a permanent Patriot missile deployment.
"We will make those decisions in consultation with the Lithuania government," he said.
The Patriot is a mobile air defence system made by Raytheon designed to intercept tactical ballistic missiles, low-flying cruise missiles and aircraft.
"Everyone knows this is not an offensive capability," Mattis said.
Moscow is holding its so-called "Zapad" drills in September that will see the Russian military showcase new hardware and upgrade existing systems in its western military region, US officials said.
Mattis later drove into the Lithuanian countryside outside Vilnius to visit a large, wooded training area about 20 kilometres (12 miles) from the border with Belarus.
A German NATO battle group along with other NATO troops is operating in the area, training to better work together and improve coordination between the militaries.
NATO is deploying the battalions in Poland and the Baltic states as tripwires against Russian adventurism in a region formerly under Moscow's control, spooked by its actions in Ukraine and Syria.
"The reason for the deployment you see right now is the lack of (Russian) respect for international law," Mattis told reporters after he visited US, Lithuanian and Dutch troops on a frigid training field.
Mattis's trip to Lithuania is his first to eastern Europe as defence secretary and his meeting with Grybauskaite, a very vocal critic of Russian leader Vladimir Putin, could be interpreted as a signal to Moscow.
The highly militarised Russian exclave of Kaliningrad is right next door and Putin is prone to sabre rattling there.
A US-led NATO battalion that is stationed nearby in Poland and the German-led battalion Mattis visited are the first line of defence for the so-called "Suwalki Gap".
NATO experts see the vulnerable stretch of territory as key to eastern flank security.
The battalions will hold exercises focused on defending the Gap from June 10-24.