We provide the catalog of X-ray detected galaxy clusters spectroscopically confirmed using DR12 SEQUELS SPIDERS spectroscopic data. Galaxy clusters were identified through the emission of their hot baryonic component as X-ray extended sources in the ROSAT All-Sky Survey (RASS). The optical counterparts were found by optimally searching photometric data for the existence of a red-sequence formed by their member galaxies. Spectroscopic redshifts obtained by SPIDERS (in the SEQUELS programme) provide definitive confirmation of the clustered nature of these objects and their redshift (up to z~0.6). We assigned cluster membership combining an algorithm and visual validation of individual objects. The gas properties derived from X-ray observations (luminosity, temperature, R200) are derived using precise cluster redshifts (Δz ~ 0.001). We compute galaxy cluster velocity dispersions using several methods adapted to the low number of spectroscopic members per system (of the order 15-40) and we show that their values correlate with cluster X-ray luminosities, within expectations. The relevant FITS file for this VAC is catCluster-SPIDERS_RASS_CLUS-v1.0.fits. This VAC was released in DR13, and contains the pilot sample of galaxy clusters confirmed in the 300 deg2 area of DR13 spectroscopy data, down to the lowest richnesses. DR14 includes a VAC that contains the sample of galaxy clusters confirmed with DR14 spectroscopic data (2500 deg2) down to richnesses λOPT = 30. Click here for more details.