The British national was released and immediately taken to a hospital, after he complained of feeling unwell, Rivers State spokesman Emmanuel Okah told VOA.
Rivers State lies in Nigeria's oil-rich Niger Delta where more than 100 oil workers have been taken hostage by militias and armed criminal gangs in the past year.
Rivers State Police Commissioner Felix Ogbaudu told VOA the man was released because he fell sick. He had no details about the man's illness.
The released Briton was snatched by unidentified assailants on January 23, along with a U.S. citizen, who also works in the oil industry. Police say they expect the U.S. worker to be released shortly.
Militia groups operating in the area say they want a bigger share of the oil wealth from the Nigerian government and companies that have pumped billions of dollars worth of crude from beneath their soil, but left residents impoverished.
Analysts say hostage taking is increasingly a moneymaking enterprise for criminal gangs with new kidnappings occurring nearly every week. Kidnappers demand tens of thousands of dollars for the safe return of foreign workers, who are generally released unharmed. Oil companies officially deny payment of ransom.
There are 29 foreign nationals held hostage in the maze of creeks and swamps that make up the Niger River Delta.
Nigeria is Africa's biggest oil producer, bringing billions of dollars of revenue into the country, yet many Nigerians exist on less than $1 a day.
Hostage taking and attacks on oil installations forced a 25 percent drop in oil exports from Nigeria last year.