Abstract The widely adopted “lamppost” thermal reprocessing model, in which the variable UV/optical emission is a result of the accretion-disk reprocessing of the highly fluctuating X-ray emission, can be tested by measuring interband time lags in quasars spanning a range of X-ray power. This work reports the interband time lag in an apparent X-ray-weak quasar, SDSS J153913.47+395423.4. A significant cross correlation with a time delay of ∼33 days (observed frame) is detected in the Zwicky Transient Facility g and r light curves of SDSS J153913.47+395423.4. The observed X-ray power seems to be too weak to account for the observed interband cross correlation with time delay. Hence the X-ray-weak quasar SDSS J153913.47+395423.4 is either intrinsically X-ray normal (but observationally X-ray weak), or the X-ray emission is not the only mechanism to drive UV/optical variability. In the former case, the required X-ray power is at least 19 times stronger than observed, which requires either an exceptionally anisotropic corona or Compton-thick obscuration. Alternatively, the corona-heated accretion-disk reprocessing or the EUV torus models may account for the observed time lags.