Monitoring antibodies in patient serum enables the diagnosis of infectious and chronic diseases, the assessment of an individual’s immune status, and possible protection against infection and could thus be a ubiquitous tool for pandemic preparedness and personalized medicine. Advancing from the traditional enzyme-linked immunosorbent assay (ELISA), a homogeneous assay format was developed that simplifies assay procedures and enables a wash-free one-step performance. SARS-CoV-2 was chosen as the model virus, and its Spike protein-derived receptor binding domain (RBD) was covalently coupled to fluorescent liposomes. The binding of patient antibodies triggered the complement system, led to liposome lysis, and allowed quantitative fluorescent detection. The liposome assay was optimized with respect to liposome lipid composition, RBD coverage and surface chemistry, incubation conditions, and pretreatment of a standardized complement source. A proof-of-principle was demonstrated through artificially supplemented anti-RBD antibodies and full titrations with known positive sera. Testing of 37 SARS-CoV-2 negative sera and 28 sera from individuals with SARS-CoV-2 (breakthrough) infections resulted in a specificity of 95%, sensitivity of 93% and an excellent correlation (R2 = 0.82, Spearman r = 0.90) with antibody titers determined in an ELISA approved for diagnostic use. Finally, the liposome assay showed a good correlation to a pseudovirus neutralization test (pVNT) (R2 = 0.72, Spearman r = 0.84), similar to the diagnostic ELISA. As the new liposome assay does not require any wash steps and can be easily adapted to other viral targets by changing the surface antigen, it provides a new avenue for high-throughput immunodiagnostics.