SUMMARYPhotosynthesis is a key process in sustaining plant and human life. Improving the photosynthetic capacity of agricultural crops is an attractive means to increase their yields. While the core mechanisms of photosynthesis are highly conserved in C3plants, these mechanisms are very flexible, allowing considerable diversity in photosynthetic properties. Among this diversity is the maintenance of high photosynthetic light‐use efficiency at high irradiance as identified in a small number of exceptional C3species.Hirschfeldia incana, a member of the Brassicaceae family, is such an exceptional species, and because it is easy to grow, it is an excellent model for studying the genetic and physiological basis of this trait. Here, we present a reference genome ofH. incanaand confirm its high photosynthetic light‐use efficiency. WhileH. incanahas the highest photosynthetic rates found so far in the Brassicaceae, the light‐saturated assimilation rates of closely relatedBrassica rapaandBrassica nigraare also high. TheH. incanagenome has extensively diversified from that ofB. rapaandB. nigrathrough large chromosomal rearrangements, species‐specific transposon activity, and differential retention of duplicated genes. Duplicated genes inH. incana,B. rapa, andB. nigrathat are involved in photosynthesis and/or photoprotection show a positive correlation between copy number and gene expression, providing leads into the mechanisms underlying the high photosynthetic efficiency of these species. Our work demonstrates that theH. incanagenome serves as a valuable resource for studying the evolution of high photosynthetic light‐use efficiency and enhancing photosynthetic rates in crop species.