Vegetation with high coverage by the alien species Mimulus guttatus was studied in the hilly regions of central Slovakia in 2015 and 2016. The floristic composition of these stands was recorded in ten phytosociological relevés corresponding to the Veronico beccabungae-Mimuletum guttati (alliance Glycerio-Sparganion) association. This association was reported for the first time in Slovakia in this study. This association was found to be closed or almost closed (mean coverage value of herb layer = 92%) and formed relatively species-rich stands (15 species per relevé), usually in narrow and small patches along the upper parts of streams and their spring areas in uplands at altitudes from 561 to 1,048 m. Localities with the Veronico beccabungae- Mimuletum guttati association were characterized by typical mountain climates, with both relatively low mean annual air temperature (5.5°C) and high mean annual precipitation (885 mm). While water temperature (~13°C) of these habitats varied considerably among streams (7.4–19.9°C), their herbaceous vegetation preferred neutral to slightly alkaline water (pH = 6.1–7.5) with low amounts of soluble mineral matter (~72.6 μS cm⁻¹). This kind of vegetation was most often developed on sites with coarser sediments (stone, gravel, and sand) formed from crystalline bedrock. A comparison of vegetation data of the Veronico beccabungae-Mimuletum guttati association across Central Europe demonstrated considerable floristic variability among regions.