Factors that affect group sizes in large ungulates are generally poorly understood for species from remote regions. Understanding grouping patterns is important for effective species management, but is lacking for the endangered Mongolian saiga (Saiga tatarica mongolica). We studied seasonal changes in the group size and social structure of saigas in relation to environmental and anthropogenic factors in western Mongolia during 2009–2012. To identify group size and composition, we observed saigas monthly while conducting monitoring surveys, and weekly while tracking radio-collared animals. We observed 9268 individuals; median group size was 6.5 (se = 1.7; range = 1–121), and groups of 1–5 animals were most common. Seasonality exerted strong effects with the smallest groups in June and largest in December. The largest mixed and nursery groups formed during pre-rutting and summer seasons, respectively, but no seasonal differences were detected for bachelor groups. The best fitting model, including Normalized Difference Vegetation Index, predation rate and season as covariates, explained ∼76% of the variation in monthly ‘typical’ group size. Our results are concordant with studies of other arid-adapted ungulates and suggest vegetation productivity, predation rate and biological cycles are responsible for saiga grouping patterns in Mongolia.