Typical many-wavelength scale of the optical fiber-integrated photonic elements (for example, ring resonators, Bragg reflectors, Mach-Zehnder interferometers, etc.) has been an insuperable obstacle for the realization of truly integrated photonic circuits that would have the dimensions compliant with the semiconductor industry standards. Doped graphene however, promises the deeply subwavelength size of the plasmonic-based optical elements due to the very short plasmon wavelength. In this work, we propose a design of the ultra-compact fiber-integrated optical switch based on the graphene-functionalized plasmonic nano-cavity for ultrafast light modulation. Presence of graphene allows to actively control the plasmonic resonance in the cavity via the electrostatic doping, so that properly tuned Fermi level in graphene results in a strong constructive (destructive) Fano interference between the propagating mode in the fiber and the graphene plasmonic mode in the nano-cavity, increasing (zeroing) the transmission efficiency at given frequency. The nano-cavity effectively works as a plasmonic Fabry-Perot resonator, significantly enhancing the coupling efficiency as well as the interference strength. Due to the strong confinement of graphene plasmons, the active volume of the switch can be as small as 10–3λ0 –3, making it possible to build an optical circuit with a very high density of elements. Furthermore, sharp profile of the Fano resonance provides a fast switching speed even with small variation of doping. Therefore, proposed design requires very low driving voltage of ~1V, while providing the modulation depth of at least 0.5.