The density of juveniles of the tri-spined horseshoe crab Tachypleus tridentatus (Leach, 1819) was measured at the lowest tide during the period from 25 to 27 July 2017 at the Sone Estuary in Kitakyushu City, Fukuoka Prefecture. It ranged from 0.063 to 1.000 indiv./m². The highest density was recorded at the mouth of Nuki River. The range of carapace width of juveniles was 17.0–60.5 mm (n = 27), corresponding to the 4th to 8th molts. No juveniles were found at the mouth of the Ono River. Median diameter of sedimentary particles at the habitat area ranged from 0.020 to 0.740 mm. Hardness, total sulfide content and ignition loss were simultaneously measured at each station, but no significant correlation was detected with the density. The median particle size was significantly different from that calculated at the same stations on 16 July 2016, suggesting that the sedimentary environments for the habitat of juveniles in Sone estuary were drastically changed within a year.