Alien species significantly change the structure and functions of green spaces in cities, can affect people's health and cause economic damage to the urban economy. Studies show that the flora of urboecosystems has been significantly transformed by human economic activity, which causes the spread and rapid advancement of invasive species and the reduction of phytodiversity. The object of the study was the grass cover of "Gagarin Park" (Dnipro). The species composition and projective cover of vascular plants of the grass layer were studied in April–May and June–July 2021–2022. Experimental sites measuring 3×3 m were laid in different parts of the park, the total number of spring descriptions – 420, summer – 230. Field work was carried out using classic geobotanical methods of research (route-reconnaissance, detailed-route, geobotanical description, establishment of test plots). 121 species of vascular plants belonging to 45 families were recorded in the grass cover of Gagarin Park (Dnipro). The most numerous are Asteraceae (17%) and Poaceae (12%). At the beginning of the growing season, the families Lamiaceae, Rosaceae, and Scrophulariaceae are characterized by the richest species composition; in the summer, the role of Aceraceae, Brassicaceae, Caryophyllaceae, Fabaceae, and Rosaceae increases. 42 adventive species belonging to 29 families were found in the grass layer of the park. The share of adventitious plants increases during the growing season from 22 to 37% in species composition. In spring, representatives of the Asteraceae, Lamiaceae, and Scrophulariaceae families predominate among the 12 registered adventives (15% each), in summer the number of adventive species increases to 36, and the Asteraceae family dominates (28%). The spectrum of life forms is dominated by herbaceous annuals - 33.3% and perennials - 23.8%, as well as numerous seedlings of woody species – 21.4%. There are much fewer biennials (9.5%) and vines (7.1%), biennials and shrubs are represented singly. American species (33.4%) are the leaders in species diversity by origin: North American and South American migroelements have 13 and 1 species, respectively. There are also numerous representatives of Mediterranean (23.8%) and Asian origin (21.4%). This ratio is more typical for the group of invasive plants than for alien species of Ukraine as a whole. The predominance of species of American origin indicates that plants that grow in similar conditions in their original habitat are best adapted to new conditions. This confirms the proposition that species with a transcontinental habitat type have greater opportunities for adaptation to new conditions than species originating from adjacent floristic regions. The spectrum of adventitious plant biomorphs in «Gagarin Park» is dominated by annuals in the spring, but in the summer the share of woody species seedlings increases. Among the life forms according to K. Raunkier (or climamorph), the largest percentage belongs to therophytes in spring, and phanerophytes and hemcryptophytes in summer. The dominant group of biomorphs is vegetatively immobile plants – 80% in spring and 55% in summer. Cenomorphic analysis of the grass cover indicates the dominance of ruderants throughout the season, which is natural for the analyzed group of adventive plants. Xeromesophytes and mesophytes predominate among hygromorphs, and mesotrophs among tromorphs, which indicates favorable conditions for the growth of tree vegetation in the park. Among the heliomorphs, heliophytes and scioheliophytes predominate, which reflects the increased level of illumination under the canopy of woody park plants compared to typical forest phytocenoses. In the grass cover of the «Gagarin park», representatives of alien species with high and medium indicators of phytocenotic activity were found. Among them, most of them belong to invasive plants and transformative species – Impatiens parviflora, Solidago canadensis, Acer negundo, Erigeron annuus, they are the ones that pose a threat to the phytodiversity of the studied ecosystem, inhibit naturalization processes and require measures to regulate the number of populations. Due to the detected increased phytocenotic activity of Partenocissus quinquefolia and Vitis vinifera, we state that these species acquire invasive properties, therefore they require the attention of researchers and monitoring.