Being a large maritime nation, the need to develop sustainable ocean planning and management processes in Portugal has been gaining increased importance in the last decade. After promulgating its first national framework law on maritime spatial planning (MSP), Portugal has recently approved a new MSP Diploma that aims at “developing” (i.e. implementing in detail) the framework law. This paper analyzes and discusses the new Portuguese MSP Diploma by (1) briefly presenting its main specificities; (2) analyzing its contents (and comparing them to the EU MSP Directive contents), namely in what pertains to environmental references; (3) analyzing the link between the EU Marine Strategy Directive (MSFD) and the MSP implementation in Portugal; and (4) discussing the main challenges that the Diploma poses to the long-term sustainability of Portuguese ocean management. Results show that environmental references represent only a small account on the Diploma contents (c. 2% against c. 5% in the EU MSP Directive); main environmental topics addressed include environmental “monitoring” and “evaluation”, “environmental protection”, “sustainability”, and “good (environmental) status”; and the ecosystem-based approach is never referred to. In Portugal the same government entity has responsibility over the implementation of both MSP and the MSFD, and such an institutional framework is expected to promote sustainable maritime uses as well as a true coordination/communication between both processes. The Diploma enshrines several “unusual” aspects that may compromise environmental sustainability. Although the new Portuguese MSP Diploma has been recently approved and promulgated, it may still be amended in the framework of a parliamentary discussion, therefore still having the opportunity to overcome the identified environmental challenges/concerns.