Biden said during a White House meeting that Finland is a "strong defence partner" involved in a "united trans-Atlantic response to holding Russia accountable" for what he called an attack on global peace.
Talks between the two leaders came as the conflict in Ukraine roused fresh concern for other European countries bordering Russia.
Finland already cooperates with NATO, but is not a member.
Ukraine's government said it wanted to seek NATO membership prior to Russian President Vladamir Putin's invasion.
The United States could declare Finland a major non-NATO ally and step up weapons sales to the country, which shares a 1,340-km border with Russia.
Last month, Finland sealed a $US9.4 billion deal to buy dozens of F-35 stealth warplanes from the United States, in a sign of the Finnish military's growing ties to NATO.
But Finland, a European Union member which was part of the Swedish kingdom until 1809 and then was under Russia's control until gaining independence in 1917, has also sought to preserve friendly relations with Moscow.
A poll by public broadcaster Yle last Monday said 53 per cent of Finns support joining, compared with 28 per cent when the Helsingin Sanomat newspaper asked the question in late January.
Finland's government has sought to calm campaigns to join the US-led defence bloc. Niinisto said in a statement that people should "keep a cool head and assess carefully the impact of the changes that have already taken place and of those that might still happen."
During a small portion of the Oval Office meeting open to reporters, Biden said his predecessor Barack Obama often said the world would be fine if they left matters up to Nordic countries.
"Well, we usually don't start wars," Niinisto replied.
