He found the 706-carat diamond, one of the 20 largest ever, in the alluvial mines in the Kono district.
Even though diamonds can be found in about a quarter of Sierra Leone, the country of about six million people is one of the world's five poorest nations, according to the United Nations.
Emmanuel Momoh found the 706 carat diamond in the eastern region of Kono.
It is being kept locked away in Sierra Leone's central bank in Freetown.
Pastor Momoh's discovery, which has not yet been valued, is the 13th-largest diamond ever found.
He has turned the diamond over to the government in hopes that it will be used to fuel development.
Despite its size, this week's discovery is considerably smaller than the Cullinan diamond, which was found in South Africa in 1905. Though Sierra Leone is known for its industry surrounding diamond mining, BBC News notes the history of that industry is highly controversial.
However, a government official in a statement on Thursday, March 16, said that the diamond will be sold with a "transparent" bidding process to the benefit of the community and country.
Abdulai Bayraytay, spokesperson of the President of Sierra Leone said: "We believe that showing it publicly would enhance our image of transparency, especially with Sierra Leone's participation in the Kimberly process, as well as open public partnership and the extractive transparency initiative in Sierra Leone".
During Sierra Leone's decade-long civil war that killed an estimated 120,000 people, the proceeds of diamond sales were often exchanged for hand guns and other weapons. Even with a blemish, the diamond could easily be worth millions.
The largest diamond ever, "Sergio", a black diamond weighing 3167 carats, was found in Brazil in 1893. That 3,106-carat stone was cut into several polished gems and the two largest pieces are part of Britain's crown jewels.
In May of 2016, Lucara, a diamond-mining fund sold an 813-carat diamond for a whopping $63 million.