jats:titleAbstract</jats:title> jats:pThe ultra-faint dwarf galaxy Reticulum II (Ret II) exhibits a unique chemical evolution history, with jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msubsup mml:mrow mml:mn72</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn12</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> </jats:inline-formula>% of its stars strongly enhanced in jats:italicr</jats:italic>-process elements. We present deep Hubble Space Telescope photometry of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the color–magnitude diagram is best fit by a model consisting of two bursts of star formation. If we assume that the bursts were instantaneous, then the older burst occurred around the epoch of reionization, forming ∼80% of the stars in the galaxy, while the remainder of the stars formed ∼3 Gyr later. When the bursts are allowed to have nonzero durations, we obtain slightly better fits. The best-fitting model in this case consists of two bursts beginning before reionization, with approximately half the stars formed in a short (100 Myr) burst and the other half in a more extended period lasting 2.6 Gyr. Considering the full set of viable star formation history models, we find that 28% of the stars formed within 500 ± 200 Myr of the onset of star formation. The combination of the star formation history and the prevalence of jats:italicr</jats:italic>-process-enhanced stars demonstrates that the jats:italicr</jats:italic>-process elements in Ret II must have been synthesized early in its initial star-forming phase. We therefore constrain the delay time between the formation of the first stars in Ret II and the jats:italicr</jats:italic>-process nucleosynthesis to be less than 500 Myr. This measurement rules out an jats:italicr</jats:italic>-process source with a delay time of several Gyr or more, such as GW170817.</jats:p>