Abstract: Objective: To investigate the effects of alginate oligosaccharides (AOS) on depression-like behavior and gut microbiota in zebrafish, a model of chronic unpredictable mild stress (CUMS). Methods: Wild-type AB strain zebrafish were randomly divided into normal, model, positive control and AOS low, medium and high dose (AOS-L, AOS-M, AOS-H) groups, with 24 fish in each group. All groups, except the normal group, received 14 d of CUMS to establish a zebrafish depression-like model. Except for the normal and model groups, which were given equal volumes of rearing water, the remaining groups were immersed in the corresponding concentrations of AOS for 60 min daily starting from day 8. After 14 d, behavioral experiments were conducted using the novel tank test (NTT) and light-dark tank test (LDB), enzyme-linked immunosorbent assay (ELISA) method was used to detect the levels of 5-hydroxytryptamine (5-HT) and norepinephrine (NE) in the brain tissues, and 16s rRNA sequencing was used to detect the changes in gut microbiota. Results: Compared with the model group, in the NTT, zebrafish in the positive control group and the AOS high dose group had significantly shorter latency for the first entry into the top, significantly longer time in the top, and significantly increased the number of entries into the top (both P＜0.01), and expanded the range of motion. Compared with the model group, in the LDB, the positive control group and the AOS high dose group significantly shortened the latency of the zebrafish’s first entry into the shiny area, the AOS medium dose group and the AOS high dose group were also able to significantly increased the duration of the time in the shiny area (P＜0.01), the positive control group and each of the AOS groups were also able to significantly increased the number of entries into the shiny area (P＜0.05 or P＜0.01). Body mass index, 5-HT and NE levels were significantly lower in the model group compared with the normal group (P＜0.01). Compared with the model group, body mass index was significantly increased in the AOS-M, AOS-H groups (P＜0.05 or P＜0.01); 5-HT levels were significantly increased in the positive control group and the AOS high dose group (P＜0.01). AOS could increase the abundance and diversity of gut microbiota and regulate the structure of community composition. At the phylum level, it regulated the relative abundance of Proteobacteria, Actinobacteriota, Fusobacteriota and Firmicutes. At the genus level, it regulated the relative abundance of genera such as Cetobacterium, Aeromonas, Plesiomonas, unclassified_f__Rhizobiaceae and Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium. Metabolic functions such as carbon metabolism, purine metabolism and pyruvate metabolism, as well as related substance transport processes in the gut were influenced. Conclusion: AOS can ameliorate depressive-like behavior and increase body mass index in CUMS zebrafish, and its effects are related to the modulation of neurotransmitters, the composition of the gut microbiota, and their metabolic and substance transport functions.