Question: Rich fens of the Sphagno warnstorfii-Tomentypnion nitentis alliance require a specific combination of base richness and climate to occur. Their rarity at the southeastern margins of their European range has previously prevented rigorous vegetation classification. We asked how many associations may be delimited here and whether some of them are restricted to the high Balkan Mountains showing high endemicity. Study area: Entire territories of Bulgaria and Romania. Methods: We compiled all available vegetation-plot records, including some hitherto unprocessed data. We classified them by both divisive (modified TWINSPAN) and agglomerative (beta-flexible clustering) numerical classification method, with OPTIMCLASS1 applied to set the number of clusters. A semi-supervised approach (k-means) was additionally applied to confirm the classification of Southern-Carpathian (Romania) rich fens, where some Balkan taxa occur. Differences in base richness and elevation were tested by one-way ANOVA with Tukey’s pairwise test. Results: Three associations were delimited and all three occur in Bulgaria, from where only one association had been previously reported. Two associations characterised by Sphagnum contortum and Balkan and Southern-European species occur in Bulgaria, but not in Romania, one at lower elevations around 1,200 m, and one at higher elevations around 2,000 m where pH is lower. One lower-elevation (around 1,300 m) association with S. warnstorfii and S. teres is shared between Romania, Bulgaria and Central Europe. Conclusions: We have described a new high-mountain association, with two subassociations that differ by successional stage and dominant peat moss species (S. contortum and S. warnstorfii, respectively). These subassociations could be reconsidered when more data from other Balkan countries are available. Rich fens in southeastern Europe are rare, have a diverse vegetation, and are deserving of the further attention of nature conservation authorities and vegetation scientists.<br> Taxonomic reference: The nomenclature was harmonized following The Euro+Med PlantBase (Euro+Med 2021) for vascular plants and Hill et al. (2006) for bryophytes, except of Angelica pancicii that is accepted as a separate taxon in Bulgaria (Andreev et al. 1992; Delipavlov et al. 2003). Critical taxa, not always reliably differentiated in the field and in literary sources, were merged to aggregates: Alchemilla vulgaris agg. (all Alchemilla species), Anthoxanthum odoratum agg. (A. alpinum, A. odoratum), Molinia caerulea agg. (M. arundinacea subsp. arundinacea, M. arundinacea subsp. freyi, M. caerulea), Palustriella commutata agg. (P. commutata, P. falcata), Plagiomnium affine agg. (P. affine, P. elatum, P. ellipticum), Sphagnum palustre agg. (S. centrale, S. palustre).<br> Syntaxonomic reference: Peterka et al. (2017) for alliances.
