Exxon said it took on 80 percent of the interest in offshore Block 13, which is situated in waters ranging from 250 to 10,000 feet. Liberian President Ellen John Sirleaf signed off on the acquisition last week.
Exxon Mobil Exploration Co. President Steve Greenlee said the government's consent is a vote of confidence for the company.
"The government of Liberia wants to develop its resources for the benefit of the country and its people," he said in a statement from Texas. "We have the capability and the expertise to help make that happen."
The company gave no indication of the reserve potential offshore Liberia. It said it will open an office in the Liberian capital within six months and start drilling offshore within three years.
Liberia is recovering politically and economically from years of civil war. The Liberian government said more than $16 billion was invested in natural resources such as oil in the past six years.
Liberia is considered frontier oil and natural gas territory.