The article discusses the changes in the scope of natural legally protected areas, that occured after the administrative reform in 1999. It indicates the tasks carried out by the Self-Government of the Mazowieckie Voivodeship in the field of regional policy and spatial management in accordance with the principle of sustainable development. The Mazowieckie Voivodeship – created under the territorial division reform – covered a system of protected areas that included the Kampinos National Park, 164 nature reserves, 9 landscape parks and 29 protected landscape areas. In nearly 25 years, 26 nature reserves and one protected landscape area were created. The boundaries of the existing forms of nature protection have also been modified. With Poland's accession to the European Union, the most significant changes concerning legally protected areas have taken place. The implementation of EU law into Polish law resulted, inter alia, in the introduction of a new form of nature protection – Natura 2000 sites, complementing the existing system of legally protected areas. By March 2023, 80 areas of the Natura 2000 network had been designated and approved in the Mazowieckie Voivodeship. The article discusses how the position and competences of the regional self-government in the field of nature protection have evolved as a result of changes in the law. The voivodeship self-government took over competences in the field of landscape parks and protected landscape areas. As a result of the acquired rights, the Mazovian Voivodeship Sejmik, inter alia, established the Mazovian Landscape Parks complex and protection plans for landscape parks. The boundaries of the protected landscape areas have been specified. Spatial databases are also maintained in the field of protected areas. Key words: Mazowieckie Voivodeship, nature conservation, legally protected areas, landscape parks, protected landscape areas, green belt