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Using a data sample of $448.1 \times 10^6$ $\psi(3686)$ events collected withthe BESIII detector at the BEPCII collider, we report the first observation ofthe electromagnetic Dalitz decay $\psi(3686) \to \eta' e^+ e^-$, withsignificances of 7.0$\sigma$ and 6.3$\sigma$ when reconstructing the $\eta'$meson via its decay modes $\eta'\to\gamma \pi^+ \pi^-$ and$\eta'\to\pi^+\pi^-\eta$ ($\eta \to \gamma\gamma$), respectively. The weightedaverage branching fraction is determined to be $\mathcal{B}(\psi(3686) \to\eta' e^+ e^-)= (1.90 \pm 0.25 \pm 0.11) \times 10^{-6}$, where the firstuncertainty is statistical and the second systematic.

Observation of $\psi(3686) \to \eta' e^+ e^- $