NATO Secretary-General Jens Stoltenberg says a change in events in Europe has prompted the response.
He referred to Russia's annexation of Crimea and its involvement in Ukraine.
Russia denies it has acted aggressively but Mr Stoltenberg says borders have been changed by force which requires a response from NATO.
"NATO defence ministers agreed on an enhanced forward presence in the eastern part of our alliance. This will be multinational, to make clear that an attack against one ally is an attack against all allies, and that the alliance as a whole will respond."
Mr Stoltenberg says the force will not be permanent, in line with an agreement with Russia that forbids the deployment of troops along its border.
The proposed solution is to rotate up to 6,000 troops through Lithuania, Latvia, Estonia, Poland, Bulgaria and Romania.
They'll be supported by a rapid-reaction force that includes air, naval and special operations units involving up to 40,000 people.
"We have increased NATO's presence in the eastern part of our alliance with enhanced air policing, maritime patrols and robust exercises. We have agreed assurance measures for Turkey with Patriot (missile) batteries, AWACS surveillance planes and an enhanced maritime presence in the eastern Mediterranean and in the Black Sea."
Britain says it will contribute two warships to NATO maritime groups this year, sending one frigate to the Baltic.
Defence Secretary Michael Fallon says being able to quickly respond to any threats is imperative.
"NATO faces pressure now on its eastern border and in the Mediterranean and we need to unite round a clear plan to deploy troops and ships to deter any aggression and the threats that we've seen. And we want to see faster deployment of those troops."
Mr Stoltenberg says he will meet Russia's Foreign Minister, Sergei Lavrov, in Germany in coming days.
He says he'll use the talks to stress that the move is a defensive measure.
But Russian Foreign Ministry spokeswoman Maria Zakharova has denounced it as destabilising.
"We do not understand what triggered those actions. We believe that they threaten not Russia itself but the strategic stability and security expected to prevail in Europe."
The full details of the NATO plan will be announced in the coming months.
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