The paper describes facts about the flora of Kostomuksha (64°35'N, 30°40' E) - one of the youngest taiga cities both nationally and globally. The flora of the city totals 432 species with 230 (53%) aboriginal species, of which 32 (14%) are apophytes, and 202 (47%) adventive species. The total number of hemerophytes (including apophytes) is 234 (54.3%). The absolute dominants among adventive species are xenophytes - 153 sp„ ergasiaphytes - 20. In terms of the establishment success the species can be grouped as follows: epoecophytes - 87 sp., colonophytes - 62, ephemerophytes - 46, agriophytes - 7. In terms of the recency of arrival kenophytes with 111 sp. (55%) dominate, and neophytes are represented by 91 sp. (45%). Native species mostly had a boreal circumpolar (29.6% of the total no of sp.) and boreal Eurasian (23.5%) distribution. The main parameters of the Kostomuksha urban flora and the flora of the nearby Kostomukshskiy strict nature reserve were compared. Comparison of the taxonomic structures of native fractions revealed a nearly complete match of the ten leading families, although the order of sequence was the same only for the first four families. When comparing the floras using Jaccard index, the greatest value (Kj = 0.64) was found for native fractions, Kj = 0.56 was the index for complete floristic lists, the lowest value (Kj = 0.35) was recorded for alien fractions. Annotated checklist of the Kostomuksha city vascular plants is included.