The brown macroalga, Sargassum horneri, is now attracting attention not only as a food source but also with regard to coast environmental preservation, coastal ecosystems, CO2 fixation, and as a marine indicator for environmental impact assessment. The present study was conducted to identify the effects of decreased seawater pH on S. horneri growth at three early developmental stages: embryo, germling, and young plant. After being exposed to acidic seawater of pH 6.0–7.5 for approximately 1–5 days, S. horneri embryos could germinate and grow when cultured in seawater with pH 8.0. At the germling stage, S. horneri growth was significantly inhibited when cultured in seawater with pH 6.0–6.6 compared to its growth in seawater with pH 8.0 (control). Moreover, at the young plant stage, S. horneri growth was significantly inhibited when cultured in seawater with pH 7.0 compared to when cultured in seawater with pH 7.5–8.0. These results demonstrate that the effects of decreased pH differ depending on the early growth stage of S. horneri, and as the growth of S. horneri progresses, the adverse effects of low pH increase. The results of this study can be used as preliminary data to promote S. horneri aquaculture in coastal areas and to assess the environmental impacts on its growth.