Tau and dark sector searches at Belle and Belle II

The Belle and Belle II experiments provide a powerful platform for beyond-the-Standard-Model searches, particularly in tau and dark sectors. This summary reports include a measurement of CP violation in ${\tau }^{-}\to {\pi }^{-}{K}_S^0{\nu }_{\tau }$, a search for ${\tau }^{-}\to {\mathrm{\ell }}^{-}\eta$, a search for axion-like particles via $e^{+}{e}^{-}\to \gamma a(\to \gamma \gamma )$ using Belle II data, and a search for feebly interacting particles in $B\to h{X}_{\mathrm{i}\mathrm{n}\mathrm{v}}$ decays using Belle data.