"The deal is worth $1 billion. We have acquired the whole shares of Hess Thailand Holdings II Limited and Hess Exploration Thailand Company Limited," chief executive Tevin Vongvanich said in a statement.
PTTEP will gain stakes in the Pailin, Morakot and Ubon mainly oil fields as well as increasing their share in two producing gas fields in the deal, the company said in a statement.
The purchase comes after PTTEP and Indonesia state-owned oil company PT Pertamina agreed last year to buy Hess's Indonesian assets for $1.3 billion.
PTTEP shares closed on Tuesday at 158.50 baht, up one baht, in trade worth 453.04 million baht.