The tea plant (Camellia sinensis (L.) O. Ktze) is an important cash crop grown worldwide. It is often subjected to environmental stresses that influence the quality and yield of its leaves. Acetylserotonin-O-methyltransferase (ASMT) is a key enzyme in melatonin biosynthesis, and it plays a critical role in plant stress responses. In this paper, a total of 20 ASMT genes were identified in tea plants and classified into three subfamilies based on a phylogenetic clustering analysis. The genes were unevenly distributed on seven chromosomes; two pairs of genes showed fragment duplication. A gene sequence analysis showed that the structures of the ASMT genes in the tea plants were highly conserved and that the gene structures and motif distributions slightly differed among the different subfamily members. A transcriptome analysis showed that most CsASMT genes did not respond to drought and cold stresses, and a qRT-PCR analysis showed that CsASMT08, CsASMT09, CsASMT10, and CsASMT20 significantly responded to drought and low-temperature stresses; in particular, CsASMT08 and CsASMT10 were highly expressed under low-temperature stress and negatively regulated in response to drought stress. A combined analysis revealed that CsASMT08 and CsASMT10 were highly expressed and that their expressions differed before and after treatment, which indicates that they are potential regulators of abiotic stress resistance in the tea plant. Our results can facilitate further studies on the functional properties of CsASMT genes in melatonin synthesis and abiotic stress in the tea plant.