Japanese plum-type (Prunus salicina Lindl. and related interspecific hybrids) production largely depends on effective cross-pollination because most cultivars exhibit gametophytic self-incompatibility. The continuous release of new cultivars from international breeding programs has increased the need to characterize their self-(in)compatibility and compatibility relationships. In this study, self-(in)compatibility and S-genotypes were analyzed in 44 recently developed Japanese plum-type cultivars, most originating from breeding programs in the United States and South Africa. Self-(in)compatibility was evaluated through microscopic observation of pollen tube growth following self-pollination, and S-genotyping combined PCR-based markers targeting the S-RNase gene and capillary electrophoresis with Sanger sequencing and NGS-based de novo assembly to resolve unassigned alleles. Of the 44 cultivars, 11 were identified as self-compatible, whereas 27 were self-incompatible, 4 showed an androsterile phenotype, and 2 yielded inconclusive responses. S-genotypes were assigned to 37 cultivars, allowing S-genotype-based compatibility relationships to be inferred for many cultivars for the first time and revealing 11 S-alleles. Three novel S-alleles (Su, Sv, and Sw) were sequence-characterized, whereas an additional previously unassigned allele profile was provisionally designated Sx based on capillary electrophoresis. These results provide a genetic and reproductive basis for identifying compatible cultivar combinations and candidate pollinizers in commercial Japanese plum orchards, although effective orchard pollination also depends on flowering synchrony and other agronomic factors. The characterization of previously undescribed S-alleles also expands the molecular resources available for breeding and future S-genotyping.