An official of the State Security Service (SSS) confirmed the release took place around 1:00 am (midnight GMT), but could not say if any ransom was paid.
The girl's captors had demanded "tens of millions of naira" for her release (tens of thousands of euros/dollars), according to state police commissioner Felix Ogbaudu.
Meanwhile police in nearby Bayelsa state said a two-year-old daughter of a boat driver was kidnapped by an unknown gang on Saturday.
"We got a report of the disappearance of a little girl yesterday. We are still investigating the incident," a police spokesman said.
He said the father was not wealthy, making it unclear whether the toddler was seized for ransom. Local newspapers said the child, Aduntunure Aweri, might have been kidnapped for ritual purposes by suspected cult gangs.
The elderly parents or small children of prominent Nigerians in the oil-rich Niger Delta region have become targets for kidnappers in recent months and oil companies have stepped up security around their employees, especially foreigners.
Since the beginning of 2006, more than 200 foreigners, mostly in the oil industry, have been taken hostages by militants in the restive region.
Most have been released unhurt after days or even weeks, usually after ransoms have been paid, although oil firms rarely admit to making such payments.