  We present the first search for the semileptonic decay $D_s^+\to \pi^0
e^+\nu_e$ using a data sample of electron-positron collisions recorded with the
BESIII detector at center-of-mass energies between 4.178 and 4.226~GeV,
corresponding to an integrated luminosity of 6.32~fb$^{-1}$. This decay is
expected to be sensitive to $\pi^0$--$\eta$ mixing. No significant signal is
observed. We set an upper limit of $6.4 \times 10^{-5}$ on the branching
fraction at the $90\%$ confidence level.
