Simple Summary The family Lucanidae Latreille, 1804, has long been considered to be one of the most archaic groups in the superfamily Scarabaeoidea and contains more than 110 genera and about 1300 extant species. Most adult lucanids display obvious sexual dimorphism as males are often larger and possess well-developed and variable mandibles used to compete with other males. For a long period of time, researchers of stag beetles have had problems and disputes, because many views are only speculation and cannot be supported by the evidence of fossil specimens. In this paper, we describe and illustrate a new fossil species from the Early Cretaceous of northeastern China: Prolucanus beipiaoensis gen. et sp. nov. Based on this well-preserved specimen, a new genus is established, representing the first record of the fossil Lucaninae in China and the oldest reliable record of the subfamily. A new stag beetle fossil is described from the Yixian Formation in northeastern China. A new genus is established based on this well-preserved specimen. Prolucanus beipiaoensis gen. et sp. nov. is firmly placed in the extant lucanid subfamily Lucaninae based on its broad prosternal process and geniculate antennae. Our discovery represents the first fossil record of Lucaninae from the Late Mesozoic of China and the oldest reliable record belonging to the subfamily. We also summarize the fossil species of stag beetles found in China and the described fossil species of the subfamily Lucaninae found in the world. A key to all fossil genera of Lucanidae from China is provided.