The large oil spill was discovered late on Sunday evening in Skarvikshamnen, one of the three harbour areas where oil and energy products are handled at Gothenburg's main port.
According to authorities, between 20 and 150 gallons of oil ended up in the sea.
Word about the spill reached emergency services at 10:45 pm on Sunday evening and the region's coastguard, firefighters and port staff worked during the night to try to prevent the oil from spreading.
"You try to barricade a portion of the spill with booms," a lead operator at the emergency services told Swedish news wire TT late on Sunday evening, referring to a tool used to contain oil spills.
The spill is now under control and the cleanup will continue throughout Monday.
Authorities at Gothenburg harbor have said they think the substance is most likely diesel, although it could be one of a number products that settle on the surface of the water.
"They have taken samples to see if it is possible to derive the source of the spill," a lead operator at the emergency services told TT.
The port's emergency services, coast guard, and various port staff working during the night to try to prevent the oil from spreading.
Good winds and currents in the right direction facilitated taking care of the oil spill.
Authorities have said the spill will not affect shipping in the area.
Skarvikshamnen is one of three zones in west Sweden where refined petroleum products, renewable energy products, chemicals and a smaller amount of crude oil are handled.
The municipality-owned Port of Gothenburg is the second largest port in the Nordic countries, after the Port of Bergen in Norway, with over 11,000 ship visits per year from over 140 destinations worldwide.
Oil spills frequently result in both immediate and long-term environmental damage to beaches, marshlands and marine ecosystems. Some of the environmental damage caused by an oil spill can last for decades after the spill occurs.