We report here about the development of a fiber optic biosensor for the detection of vitamin D. The sensing platform is based on a long period grating (LPG) inscribed into a specialty double cladding fiber, whose sensitivity is enhanced through the mode transition phenomenon by chemical etching of the fiber outer cladding. The LPG is further covered with a nanosized layer of graphene oxide, providing carboxylic functional groups for the covalent immobilization of a vitamin D specific antibody. The testing was conducted using the major circulating form of vitamin D3, i.e., 25-OH-D3 (molecular weight ~400 Da), for which concentrations within 10-1000 ng/mL in buffer solution were measured. A limit of detection of medical interest lower than 10 ng/mL was attained.