A flexible biosensor based on an olfactory receptor/hydrogel nanostructure has been developed for directly detecting odorants in both the gas and liquid phases. Here, nanodiscs containing olfactory receptors were encapsulated in an alginate hydrogel, and then, a nanodisc/hydrogel hybrid structure was attached to a carbon nanotube field-effect transistor (CNT-FET). The porous hydrogel enables odorant molecules to diffuse into the hybrid structure and bind to embedded nanodiscs, leading to conductance changes in the underlying CNT channel. Therefore, our sensor could sensitively detect odorant molecules down to concentrations of 1 pM. The biosensors also discriminated octanal from non-target molecules with high selectivity. Moreover, we demonstrated that our biosensor could detect octanal molecules in the gas phase, as well as in the solution phase, and be reused up to 5 times without significant performance degradation. As a proof of concept, the biosensor device was fabricated on polyethylene terephthalate (PET) film and used to build a flexible bioelectronic nose platform. These results show that our flexible biosensor based on a nanodisc/hydrogel hybrid structure could be a useful tool for the evaluation of food flavors in the gas or solution phase and could be used in versatile applications such as wearable devices.