Heavily doped tungsten oxide nanoparticles with localized surface plasmon resonances (LSPRs) were recently highlighted as potential substitutes for noble metals in the field of plasmonic applications. Herein, oxygen deficient, spherically shaped WO_3−x nanocrystals (NCs) were synthesized with a pronounced visible LSPR absorbance peak instead of a broadband tail usually observed for WO_3−x nanowires. Although the tuning of the plasmon resonances was achieved mainly by changing the nanocrystals composition or solvent refraction index, we demonstrate this via the interfacial charge donation/extraction. In an aqueous NCs dispersion, the LSPR peak was either blue shifted in an acidic solution up to 80 nm or bleached by a basic solution making the NCs appropriate for sensing applications.