Stoltenberg also confirmed that NATO will hold a video summit on Friday to discuss the Russian invasion of its pro-Western neighbour.
And he reiterated that NATO had no "plans" to send alliance troops to Ukraine.
It is the first time the alliance has publicly said it is activating its defence plans, which were drawn up after Russia's 2014 invasion and annexation of Ukraine's Crimea peninsula.
Stoltenberg did not give details of them beyond saying they are "defensive plans" allowing deployments that "cover the whole east of our alliance" and which "give our military commanders some more authority within politically defined guidelines".
He said it would include elements of NATO's rapid reaction force of 40,000 soldiers, including a highly prepared unit of 7,000 personnel, most of them French, and an air wing under French command.
Stoltenberg said Friday's summit would also include non-NATO members Sweden and Finland, and EU chiefs Ursula von der Leyen and Charles Michel.
The NATO chief said the invasion would have "long-term effects" on the Western alliance's relationship with Russia and NATO's security posture.
"We don't have all the answers today. But it will be a new reality. It will be a new Europe after the invasion we saw today," he said.
Russia, he said, had not taken "seriously" efforts to find a political solution to the tensions that preceded its military attack on Ukraine.
"So Russia has shut the door to a political solution. We regret that. But that's, sadly, the reality, which has severe and very serious consequences for the people of Ukraine, but also actually impacts the security for all of us.
"And that's the reason why we step up our presence in the eastern part of the alliance."
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