According to the portal, the incident took place 50 nautical miles southeast of Nigeria's Agbami Oil Terminal at 03:00 GMT. The vessel in question is the Maltese-flagged MT New Ranger chemical tanker.
"The crew are reportedly mustered in the citadel, with 4 perpetrators onboard. The incident is ongoing", the portal wrote.
On Saturday, the portal also reported that the New Ranger was boarded by pirates. The incident took place 220 nautical miles to the south of the Nigerian capital, Lagos.
The maritime security portal on 11 November set the risk rating for the Gulf of Guinea region to critical amid a surge in pirate attacks.