Abstract Objectives Antiphospholipid syndrome (APS) is a prothrombotic condition defined by recurrent thrombosis, pregnancy complications and circulating antiphospholipid antibodies (aPL), including anti-β2-glycoprotein I (β2-GPI). In clinical practice it is possible to find patients with APS persistently negative for the aPL tests according to Sydney criteria (‘seronegative APS’, SN-APS). Recently, several autoimmune responses have been described as a consequence of post-translational modifications of their target autoantigens. This study was undertaken to test carbamylated-β2-GPI (Carb-β2-GPI) as a new autoantigen of APS. Methods β2-GPI was carbamylated by potassium cyanate and used to investigate its effect on monocyte-derived dendritic cell (moDC) phenotype and function. Sera from 114 SN-APS patients, 60 APS, 20 patients with RA, 20 non-APS thrombosis and 50 healthy donors were analysed for anti-Carb-β2-GPI by ELISA. Results Carb-β2-GPI is able to activate moDCs, inducing upregulation of CD80, CD86 and CD40, activation of extracellular signal-regulated kinase, p38 mitogen-activated protein kinase and nuclear factor-κB, and IL-12p70 release. Serological results showed that both 37/114 SN-APS (32.46%) and 23/60 APS (38.33%) patients resulted positive for anti-Carb-β2-GPI. Interestingly, SN-APS patients who tested positive for anti-Carb-β2-GPI showed a higher prevalence of thrombocytopenia (P = 0.04, likelihood positive ratio of 3.9). Conclusion Data obtained from both functional tests on moDCs and immunological approaches prompted identification of Carb-β2-GPI as a ‘new’ antigenic target in APS. In particular, anti-Carb-β2-GPI revealed a potential usefulness in identification of a significant proportion of SN-APS patients. Moreover, since patients who tested positive for anti-Carb-β2-GPI reported a high risk of thrombocytopenia, this test may be considered a suitable approach in the clinical evaluation of SN-APS.