Since 1991, numerous localities of rare, threatened and protected vascular plant species in the southern part of the Mazowiecki Landscape Park and its vicinity have been recorded but not yet published. This paper describes the localities and distribution of 24 species, including 13 species red-listed in Poland and 14 species under legal protection in Poland, the most valuable being Hierochlöe odorata, Pulsatilla pratensis, Ophioglossum vulgatum and Dianthus superbus. Four species were previously recorded in the Mazowiecki Landscape Park area: Gagea pratensis, Hierochlöe odorata, Stellaria longifolia and Veronica teucrium. In 2017–2018 a survey was carried out in order to confirm the previously known localities of the most interesting species and to find new ones. Many of the previously recorded localities are no longer extant (e.g. P. pratensis, V. teucrium). The extinction of rare vascular plant species in the Mazowiecki Landscape Park is proceeding dynamically and needs to be thoroughly documented.