They say Maj Gen Nelson Williams, who is now Sierra Leone's Deputy High Commissioner to Nigeria, is back at his base in the capital Abuja.
It was not immediately clear whether any ransom was paid for his release.
He was seized on Friday as he was travelling to the northern state of Kaduna for a military ceremony.
"He is in sound health," Nigerian police spokesman Don Awunah was quoted as saying by the AFP news agency.
Maj Gen Williams is believed to be the first Sierra Leone diplomat to be abducted anywhere in the world since the country became independent in 1961.
His seizure was hugely embarrassing for the Nigerian government, correspondent say.
Kidnapping for ransom is common in some parts of Nigeria.