Embryos within egg capsules often suffer mortality risk from abiotic and biotic factors, and embryos have a few options available for their defense. Altering timing of hatching is one of the defensive behaviour against egg mortality risk. However, inappropriate hatching timing such as early hatching may incur a cost. Therefore, embryos within egg capsules need accurate information about their egg-mortality risk to adjust their hatching timing. To examine whether embryos adjust hatching timing in response to egg mortality risk, I exposed embryos within egg capsules of a frog, Buergeria japonica, to different salinity levels. Salinity is a major mortality risk for amphibian eggs, and eggs of this frog are exposed to unexpected salinity fluctuation because spawning occurs in coastal areas. When exposed to high salinity, embryos of B. japonica hatched earlier than those exposed to low salinity. Especially, when transferred to salinity levels of 5‰ or more, embryos hatched almost immediately. Hatchings that emerged earlier showed small body size, earlier developmental stage, and low swimming performance. The present study demonstrated that salinity is one of triggers that induce early hatching in amphibians, and embryos of B. japonica adjust hatching timing based on salinity differences accordingly.