Abstract Objectives Onset of primary SS is usually between 40 and 60 years of age, with severe systemic complications in 15% of cases. We sought to determine whether early-onset disease is related to a specific phenotype and if it is predictive of a poor outcome. Methods Biological and clinical data from 393 patients recruited in the ASSESS cohort, a French multicentre prospective cohort, were compared according to age at diagnosis. Results Fifty-five patients had early-onset disease, defined as age ⩽35 years at diagnosis, and presented a significantly higher frequency of salivary gland enlargement (47.2% vs 33.3%, P = 0.045), adenopathy (25.5% vs 11.8%, P = 0.006), purpura (23.6% vs 9.2%, P = 0.002) and renal involvement (16.4% vs 4.4%, P = 0.003). They had a higher frequency of hypergammaglobulinaemia (60.8% vs 26.6%, P < 0.001), RF positivity (41.5% vs 20.2%, P < 0.001), low C3 level (18.9% vs 9.1%, P = 0.032), low C4 level (54.7% vs 40.2%, P = 0.048) and autoantibodies 84.6% with anti-SSA vs 54.4% (P < 0.001) and 57.7% with anti-SSB vs 29.7% (P < 0.001). The change in ESSDAI scores between baseline and the 5-year follow-up was significantly different (P = 0.005) with a trend for worsening in the early-onset group (0.72, P = 0.27) and a significant improvement in the later onset group (−1.27, P < 0.0001). Conclusion Early-onset primary SS is associated with a specific phenotype defined by clinical and biological features known to be predictive factors of severe systemic disease. Interestingly, we showed a different evolution of the ESSDAI score depending on the age at disease onset, patients with early-onset disease tending to worsen over time.