The approximately 25 tonnes of oil were however not expected to cause any major environmental damage as low winds and currents were predicted for the next two days.
“This is definitely something we can clean up,” coast guard official Magnus Söderberg told AFP.
It is not yet known where the oil came from.
The slick, located between the islands of Gotland and Öland, was discovered on Monday during a routine flight over the area, Soederberg said.
Three coast guard vessels were en route to the scene on Monday afternoon.