The dark sector provides a broad class of possibilities for physics beyond the Standard Model, yet no convincing signal has been found in searches for weakly interacting massive particles. This has increased the interest in the sub-GeV dark sector, which is difficult to reach with conventional direct-detection experiments. The BESIII experiment, operating in the $\tau$-charm region, combines a clean $e^+e^-$ environment, large samples of $J/\psi$, $\psi(3686)$ and $D\bar D$ events, and powerful single-tag/double-tag techniques, providing an ideal platform for such searches. This contribution summarizes recent BESIII searches for dark-sector and invisible particles: a massless dark photon in $D^0\to\omega\gamma'$ and $D^0\to\gamma\gamma'$, a dark scalar in $\eta\to\pi^0+\mathrm{invisible}$, an invisible particle in $J/\psi\to\phi+\mathrm{invisible}$, invisible $K_S^0$ decay, a dark baryon in $\Xi^-\to\pi^-\chi$, and a light vector boson in $\chi_{cJ}\to J/\psi X$. No significant signal is observed in these searches. The resulting upper limits constrain dark-sector couplings, dark-matter interactions and effective operators, highlighting the complementary sensitivity of BESIII to the sub-GeV dark-sector frontier.