It says the fighter performed a barrel roll plane over the American plane.
It is the second incident in the Baltic this month in which the US has accused Russian planes of flying aggressively.
Two Russian planes flew close to a US guided missile destroyer almost a dozen times in the Baltic on 13 April.
"There have been repeated incidents over the last year where Russian military aircraft have come close enough to other air and sea traffic to raise serious safety concerns, and we are very concerned with any such behaviour," Pentagon spokesman Daniel Hernandez said on Friday.
"The US aircraft was operating in international airspace and at no time crossed into Russian territory.  This unsafe and unprofessional air intercept has the potential to cause serious harm and injury to all air crews involved.
"More importantly, the unsafe and unprofessional actions of a single pilot have the potential to unnecessarily escalate tensions between countries."
Mr Hernandez said the Su-27's "erratic and aggressive manoeuvres" also threatened the safety of the US aircrew, coming within 7.6m (25ft) of the fuselage of the American plane before conducting its barrel roll.
Russian Foreign Minister Sergei Lavrov on Friday defended the flybys of warplanes over the US Navy destroyer in the Baltic Sea, insisting they were only looking at the ship "from a safe distance".
Military encounters between Russia and the US and its allies have escalated significantly over the past two years, ever since Russia's annexation of Crimea and the breakdown of relations between East and West.
The BBC's Gary O'Donoghue in Washington reported after the destroyer incident that Russia's actions were regarded by defence analysts as a flexing of muscle - a reminder that Russia has military might and cannot be pushed around.
But our correspondent says the frequency of such situations means many fear that a full-on confrontation - be it deliberate or accidental - is just a matter of time between the world's two great military powers.
A Russian jet fighter that intercepted a US Air Force reconnaissance plane on Friday did so in an "unsafe and unprofessional manner" over the Baltic Sea, the Pentagon has said.