The primitive deer (subfamily Lagomerycinae) Lagomeryx and Stephanocemas are characterized primarily by their palmate antlers. Two lagomerycines, Lagomeryx manai, sp. nov., and Stephanocemas rucha, are described for the first time from Q and K coal layers of the late middle Miocene (13.4–13.2 Ma) Mae Moh Basin in northern Thailand. A species-level phylogeny of the Ligeromeryx-Lagomeryx clade, based on cranial appendages, reconstructs Lagomeryx manai, n. sp., as a derived species of Lagomeryx, sister group of Lagomeryx complicidens. This study suggests that the large species of Lagomeryxare restricted geographically to Asia and dispersed to Southeast Asia at the latest during late middle Miocene, where they are represented by Lagomeryx manai, n. sp. The paleoenvironmental studies of five Mae Moh mammalian taxa, a cervid (Lagomeryx manai, n. sp.), an indeterminate bovid, a suid (Conohyus thailandicus), a rhinoceros (Gaindatherium sp.), and a proboscidean (Stegolophodon sp.), investigated with stable carbon and oxygen isotope analyses of tooth enamel, indicate that the Mae Moh mammals inhabited a wide range of habitats from woodlands to grasslands in a C3-plant-dominated environment. The new species of Lagomeryx seems to have been living in an open environment, contrary to its European relatives. The serial isotopic samples also support that Mae Moh herbivores probably lived in a low-seasonal climate during the late middle Miocene of northern Thailand.