  Using a sample of $(27.12\pm 0.14) \times 10^{8}$ $\psi(3686)$ events
collected with the BESIII detector, the decay $\psi(3686) \to \Lambda
\bar{\Lambda} \eta'$ with $\eta'$ subsequently decaying into $\gamma \pi^+
\pi^-$ and $\eta \pi^+ \pi^-$ is observed for the first time. The branching
fraction of $\psi(3686) \to \Lambda \bar{\Lambda}\eta'$ is measured to be
$(7.34\pm0.94(stat.)\pm0.43(sys.))\times10^{-6}$. No resonant structures are
evident in the $\Lambda\eta'$, $\bar{\Lambda}\eta'$ and $\Lambda\bar{\Lambda}$
mass spectra.
