This article addresses the royal forests in Portugal during the Union of Crowns within the field of environmental political history. Firstly, it sheds light upon the royal forests by looking at the forestry legislation issued from 15th to 17th centuries. Secondly, it aims to contribute to the deforestation debate during the kingships of Philip II and III of Portugal that resulted from imperial demands. During these kingships the deforestation was deployed as a founding concept both to justify and to discredit the drastic forestry policies taken by the crown, which pursued to reduce, rationalize and protect the woodlands. In this article it is used the term “administrative deforestation” as the measures adopted by the Spanish Monarchy were more likely due to a political-administrative crisis of the Portuguese crown rather to a critical shrinking of the royal forests because of shipbuilding.