Early diagnosis and monitoring of SARS-CoV-2 virus is essential to control COVID-19 outbreak. In this study, we propose a promising surface enhanced Raman scattering (SERS)-based COVID-19 biosensor for ultrasensitive detection of SARS-CoV-2 virus in untreated saliva. The SERS-immune substrate was fabricated by a novel oil/water/oil (O/W/O) three-phase liquid-liquid interfaces self-assembly method, forming two layers of dense and uniform gold nanoparticle films to ensure the reproducibility and sensitivity of SERS immunoassay. The detection was performed by an immunoreaction between the SARS-CoV-2 spike antibody modified SERS-immune substrate, spike antigen protein and Raman reporter-labeled immuno-Ag nanoparticles. This SERS-based biosensor was able to detect the SARS-CoV-2 spike protein at concentrations of 0.77 fg mL−1 in phosphate-buffered saline and 6.07 fg mL−1 in untreated saliva. The designed SERS-based biosensor exhibited excellent specificity and sensitivity for SARS-CoV-2 virus without any sample pretreatment, providing a potential choice for the early diagnosis of COVID-19. Display omitted •A new SERS-based COVID-19 biosensor was developed for ultrasensitive detection of SARS-CoV-2 virus.•A novel three-phase interfaces self-assembly method was applied for fabricating the SERS-immune substrate.•The SERS-immune substrate assembled by highly dense and uniform Au NPs can improve the reproductivity of SERS immunoassay.•The biosensor can detect the SARS-CoV-2 spike protein at ultra-low concentration of 6.07 fg mL−1 in untreated saliva.•The biosensor exhibits excellent specificity and sensitivity for SARS-CoV-2 virus without any sample pretreatment.