The agreement -- to develop the M9 block in the Gulf of Martaban -- was signed by PTT Exploration and Production (PTTEP) with Myanmar's junta in the country's remote capital Naypyidaw on Monday, the official told AFP.
The official declined to reveal the value of the deal, but Thailand's The Nation newspaper reported it would entail an investment of about two billion dollars.
Under the deal, PTTEP will work with the state-run Myanmar Oil and Gas Enterprise to develop the field, the Thai firm's CEO Anon Sirisaengtaksin said in a statement to the stock exchange.
The field is expected to produce 300 million cubic feet of gas per day, with 80 percent exported to Thailand and the rest supplying Myanmar, the statement said.
Thai energy minister Poonpirom Liptapanlop said in a statement that the deal "is very important and beneficial for Thailand to obtain natural gas to supply its rising energy needs, and to enhance the country's long-term energy security."
Energy-hungry Thailand imports about 20 percent of its gas from Myanmar and is vying for a bigger share of its vast natural resources.
State media in Myanmar previously estimated the M9 block contained 8.0 trillion cubic feet of gas.
Myanmar, one of the world's poorest nations, is under a series of US and European economic sanctions imposed over the junta's human rights abuses and its crackdown last year on pro-democracy protests.
But the impact of the sanctions has been weakened as neighbours such as China, India and Thailand spend billions of dollars for a share of Myanmar's energy resources to solve energy problems at home.
According to 2006 official figures, 13 foreign oil companies are working on 33 projects in the country.