An antibody- and label-free detection of a phosphoprotein (α-casein) has been achieved using an organic field-effect transistor (OFET)-based sensor. The fabricated OFET device possesses an extended-gate electrode functionalized with an artificial phosphoprotein receptor (Zn(II)-dipicolylamine complex, Zn(II)-DPA). It is shown that the OFET responds to the molecular recognition processes involving the Zn(II)-DPA at the extended-gate electrode. The binding of α-casein to the receptor in a HEPES solution results in an analyte-specific changes of the drain current of the OFET. The successful demonstration of the antibody- and label-free detection using the OFET could pave the way to the development of low-cost, disposable, and portable electronic sensor devices.