We present an approach for the label-free detection of cytokine biomarkers using an aptamer-functionalized, graphene-based field effect transistor (GFET) nanosensor on a flexible, SiO2-coated substrate of the polymer polyethylene naphthalate (PEN). The nanosensor conforms to the underlying nonplanar surface and performs GFET-based rapid transduction of the aptamer–biomarker binding, thereby potentially allowing the detection of cytokine biomarkers that are sampled reliably from human bodily fluids (e.g., sweat) in wearable sensing applications. In characterizing the suitability of the nanosensor for wearable applications, we investigate the effects of substrate bending on the equilibrium dissociation constant between the aptamer and the biomarker as well as the graphene transconductance. The utility of the nanosensor is demonstrated by the detection of tumor necrosis factor-α (TNF-α), an inflammatory cytokine biomarker. Experimental results show that the flexible nanosensor can specifically respond to changes in the TNF-α concentration within 5 minutes with a limit of detection as low as 26 pM in a repeatable manner.