Around 8,600 troops from 18 nations are taking part in the annual BALTOPS naval exercise.
It aims to demonstrate how NATO can defend itself against any adversary.
The 47th edition involves maritime, air and ground forces with around 50 ships and submarines, and 40 aircraft.
The training includes finding and destroying sea mines and submarines, the use of air defence and landing troops onshore as well as defence against attack from enemy navy vessels.
NATO spokesperson Oana Lungescu said: "BALTOPS is now in its 47th year and is not directed against anyone – but clearly the security environment in the region has deteriorated after Russia’s illegal annexation of Crimea.”
It is the largest exercise series in the Baltic Sea.
This year’s BALTOPS is run by the US Navy’s 2nd Fleet command in Norfolk, Virginia and is the command’s first major engagement in Europe.
Troops come from the United Kingdom, Belgium, Denmark, Estonia, Finland, France, Germany, Latvia, Lithuania, the Netherlands, Norway, Poland, Portugal, Romania, Spain, Sweden, Turkey, and the United States.