As a dietary supplement for humans and animals, Astaxanthin (Ax) is widely believed to possess antioxidant and anti-inflammatory properties. In this study, we attempted to evaluate the protective effects of Ax on dextran sodium sulfate (DSS)-induced colitis in mice and the underlying molecular mechanism. Our results suggested that Ax significantly reduced the severity of DSS-induced colitis in mice, as evidenced by increased colon length, decreased disease activity index (DAI), and attenuated inflammatory factors. In addition, Ax significantly increased the diversity of gut microbiota in mice with colitis, remodeled the microbial composition, promoted the production of beneficial bacteria (e.g., Lactobacillaceae), and inhibited the production of harmful bacteria (e.g., Lachnospiraceae and Muribaculaceae). In conclusion, Ax alleviated DSS-induced colitis by maintaining the intestinal barrier and regulating intestinal microbes.