Display omitted •A transparent CNT immunosensor is presented for detection of a prostate cancer biomarker osteopontin.•This immunosensor showed a highly linear and reproducible behavior from 1pgmL−1 to 1μgmL−1.•The limit of detection of the immunosensor was 0.3pgmL−1.•This immunosensor demonstrated high selectivity against bovine serum albumin and human serum. Osteopontin (OPN) is involved in almost all steps of cancer development, and it is being investigated as a potential biomarker for a diagnosis and prognosis of prostate cancer. Here, we report a label-free, highly sensitive and transparent immunosensor based on single-walled carbon nanotubes (SWCNTs) for detection of OPN. A high density of COOH functionalized SWCNTs was deposited between two gold/indium tin oxide electrodes on a glass substrate by dielectrophoresis. Monoclonal antibodies specific to OPN were covalently immobilized on the SWCNTs. Relative resistance change of the immunosensors was measured as the concentration of OPN in phosphate buffer saline (PBS) and human serum was varied from 1pgmL−1 to 1μgmL−1 for different channel lengths of 2, 5, and 10μm, showing a highly linear and reproducible behavior (R2>97%). These immunosensors were also specific to OPN against another test protein, bovine serum albumin, PBS and human serum, showing that a limit of detection for OPN was 0.3pgmL−1. This highly sensitive and transparent immunosensor has a great potential as a simple point-of-care test kit for various protein biomarkers.