A novel electrochemical immunosensor for the detection of the important marine biotoxins domoic acid (DA) and okadaic acid (OA) was developed. The sensors used carbon black modified screen-printed electrodes (CB-SPE) obtained using a high-throughput method. The electrochemical performance and stability of CB modified SPEs and bare carbon SPEs (c-SPEs) were compared using cyclic voltammetry and electrochemical impedance spectroscopy. CB-SPEs showed improved long term (at least six months) stability and electro-catalytic properties compared with c-SPEs. The CB-SPEs were bio-functionalized with DA or OA protein-conjugates and used to develop two indirect competitive immunosensors using differential pulse voltammetry (DPV). The DPV signals obtained for the OA and DA immunosensors fitted well to four-parameter dose-response curves (R2>0.98) and showed excellent LODs (LOD=1.7 ng mL-1for DA in buffer; LOD=1.9 ng mL-1for DA in mussel extract; LOD=0.15 ng mL-1for OA in buffer; LOD=0.18 ng mL-1for OA in mussel extract). No significant interference of the naturally co-occurring marine toxins saxitoxin, tetrodotoxin and OA was detected for the DA immunosensor. Similarly, for the OA immunosensor saxitoxin, tetrodotoxin and DA did not cross-react and very limited interference was observed for the dinophysis toxins DTX-1, DTX-2 andDTX-3 (OA congeners). Moreover, both immunosensors remained stable after at least 25 days of storage at 4 °C. This work demonstrates the potential of affordable, mass-produced nanomaterial modified SPEs for marine toxin detection in shellfish.