This　work　shows　the　electrical　switching　of　the　nonparametric　optical　nonlinearity　of　tungsten　oxide　(WO3−x).　The　sign　and　magnitude　of　the　effective　nonlinear　absorption　coefficient　(βeff)　can　be　modulated　via　application　of　an　external　bias.　With　laser　excitation　at　1030 nm,　WO3−x　shows　a　relatively　large　saturable　absorption　(SA)　under　an　applied　voltage　(VA)　of　−2.5 V,　with　βeff　being　as　large　as　−632 cm　GW−1,　while　reverse　saturable　absorption　(RSA)　is　found　for　VA　larger　than　−1.5 V.　The　electrical　switching　of　the　nonlinear　optical　(NLO)　response　is　reproducible　and　durable.　Both　electrostatic　and　electrochemical　dopings　of　WO3−x　occur　during　VA　variation,　with　SA　resulting　mainly　from　the　electrochemical　doping　(the　intercalation　of　H+　into　the　lattice　of　WO3−x).　The　wavelength-dependent　NLO　performance　of　pristine　WO3−x　is　attributed　to　competition　between　one-photon　absorption　and　two-photon　absorption,　while　the　VA-derived　NLO　response　is　correlated　with　variation　in　the　band　structure　and　its　population.　These　results　suggest　a　promising　approach　for　the　postsynthesis　modulation　of　the　NLO　response　and　a　potential　device　configuration　for　further　optoelectric　applications.　©　2021　Wiley-VCH　GmbH