Search for<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi><mml:mi>P</mml:mi></mml:math>violation in the decay<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>τ</mml:mi><mml:mo mathvariant="bold">−</mml:mo></mml:msup><mml:mo>→</mml:mo><mml:msup><mml:mi>π</mml:mi><mml:mo mathvariant="bold">−</mml:mo></mml:msup><mml:msubsup><mml:mi>K</mml:mi><mml:mi>s</mml:mi><mml:mn>0</mml:mn></mml:msubsup><mml:mo mathvariant="bold" stretchy="false">(</mml:mo><mml:mo mathvariant="bold">≥</mml:mo><mml:mi>O</mml:mi><mml:msup><mml:mi>π</mml:mi><mml:mn>0</mml:mn></mml:msup><mml:mo mathvariant="bold" stretchy="false">)</mml:mo><mml:msub><mml:mi>ν</mml:mi><mml:mi>τ</mml:mi></mml:msub></mml:math>

We report a search for $CP$ violation in the decay ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{K}_{S}^{0}(\ensuremath{\ge}0{\ensuremath{\pi}}^{0}){\ensuremath{\nu}}_{\ensuremath{\tau}}$ using a data set of $437\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $\ensuremath{\tau}$-lepton pairs, corresponding to an integrated luminosity of $476\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, collected with the BABAR detector at the PEP-II asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ storage rings. The $CP$-violating decay-rate asymmetry is determined to be $(\ensuremath{-}0.36\ifmmode\pm\else\textpm\fi{}0.23\ifmmode\pm\else\textpm\fi{}0.11)%$ approximately 2.8 standard deviations from the standard model prediction of $(0.36\ifmmode\pm\else\textpm\fi{}0.01)%$.