A spokesperson for major energy commodities trading company Gunvor said an "incident" occurred during maintenance work on one of its storage tanks.
A police spokesperson said that there was no immediate sign of sabotage and that all possible scenarios were under investigation.
The port of Rotterdam is the largest in Europe and a major hub for crude oil and fuel imports and storage.
It hosts Europe's largest oil refinery, Shell's 404,000-barrel-per-day Pernis plant, in addition to other oil refineries operated by BP, ExxonMobil and Vitol.
The explosion occurred about 11.30am on Thursday.
Gunvor said it was co-operating with local authorities.
Shell said its operations at the port were unaffected by the blast.
A Rotterdam Port spokesperson said LNG terminals were not affected by the blast.
Data from European gas transmission group ENTSOG showed a temporary drop in gas sendout from Rotterdam's Gate LNG terminal, and then volatile flows.
It was not clear if that was a consequence of the blast or an unrelated power outage elsewhere in the sprawling port.
Any prolonged disruption would be closely watched by traders, as Gate is one of northwest Europe's key LNG import hubs and Dutch gas storage sites are currently only about 40 per cent full, according to Gas Infrastructure Europe data, ahead of the northern hemisphere winter heating season.
ExxonMobil said the power outage had been resolved and that its operations were normal at its 191,000-barrel-per-day refinery, declining to confirm an alert from Wood Mackenzie seen by Reuters that all monitored units had shut down.
Rotterdam Port said there was no connection between the blast and the power outage.
The port had experienced several power outages earlier in the day but none were connected to the blast, the local safety authority said.
