The CLAAS-2.1 record provides cloud properties derived from the SEVIRI sensor onboard METEOSAT second generation (MSG) satellites. This first update of the second edition (Benas et al., 2016; CLAAS-2 DOI:10.5676/EUM_SAF_CM/CLAAS/V002; see also Stengel et al., 2014) features a temporal extension by two years (the record is now covering 2004-2017) and a bugfix for the monthly CTO products. As for previous editions of CLAAS, the solar SEVIRI channels of MSG-1, MSG-2 and MSG-3 were intercalibrated with MODIS Aqua (following Meirink et al., 2013) before applying the cloud retrievals. CLAAS-2.1 features the following cloud properties: cloud mask/type, cloud top temperature/pressure/height, cloud phase as well as cloud microphysical properties such as optical thickness, effective droplet radius and cloud water path. The data are available on native SEVIRI resolution, i.e. 15 minutes repeat cycle and 3km (nadir) to 11km (edge of the field of view) spatial resolution. In addition, spatio-temporal averages of the above mentioned cloud properties are included: Daily and monthly averages and monthly histograms on a 0.05° x 0.05° grid as well as monthly mean diurnal cycles on a 0.25° x 0.25° grid. The results of the comprehensive evaluation conducted for CLAAS-2 which were reported in Benas et al. (2016), remain valid for CLAAS-2.1. Along with the data, updated documentation including user guide, algorithm descriptions, reprocessing layout and extensive validation studies, is provided. With CLAAS-2.1, regional and large scale cloud processes at temporal scales of minutes to years can be studied. SEVIRI-based surface radiation products, which were part of CLAAS in edition 1, are released in a separate dataset (SARAH-2.1). This is a Thematic Climate Data Record (TCDR).