Updated project metadata. Meiosis, a reductional cell division, relies on precise initiation, maturation and resolution of crossovers (COs)-connections between homologs chromosomes-to ensure their accurate segregation during metaphase I. This process is finely regulated in eukaryotes by the interplay of RING-E3 ligases such as Rnf212 and Hei10 in mammals. In this study, we functionally characterized a novel RNF212B E3 ligase. RNF212B co-localizes and interacts with RNF212 forming numerous small foci at zygotene. These consolidate into larger foci at mature COs, colocalizing with Hei10 and MLH1 at pachytene on a synapsis-dependent and DSB-independent manner. Moreover, Rnf212b interacts with pro-COs proteins such as Tex11, MSH4 and RPA and other recombination proteins. Genetically, RNF212B foci depend on the presence of RNF212 and MSH4 but not on Hei10 and CNTD1. Rnf212b-deficient and -enzymatic-dead mutant mice exhibit modest synapsis defects a reduction in pro-CO factors (MSH4, TEX11, RPA, MZIP2) and a complete loss of MLH1-staining of COs, resulting in metaphase I with most univalents. Double mutants for RNF212b and RNF212 exhibit an identical phenotype, while double heterozygous demonstrate a dosage-dependent reduction in the number of COs relative to the single mutant, indicating a functional interplay between both paralogs. SUMOylome analysis of testis from Rnf212b mutants and pull-down analysis of Sumo- and Ubiquitin-tagged Hela cells, suggest that RNf212B is a novel E3 ligase with Ubiquitin activity, serving as a crucial factor for CO designation and maturation. This pathway holds physiological significance, with RNF212B and RNF212 genetic variants substantially contributing to the natural variation in genome-wide recombination rates in humans and mammals.