Background: This study was conducted to investigate the effects of different levels of L-β-galactoglucan on growth performance, palatability, and health condition of dogs. Methods: A total of 32 healthy beagle dogs (2.0 ± 0.5 yr; 13.2 ± 2.1 kg) were randomly assigned into four treatment groups, with 8 dogs in each group. The dogs were fed basal diets supplemented with 0 (control), 0.25, 0.5, or 1% L-β-galactoglucan. Results: The results showed that the feed intake ratio of the dogs in the Low_Gal (0.25%) group was significantly higher (p < 0.05) as compared with the control (Con) group. The low-density lipoprotein cholesterol (LDL-C) levels of the Mid_Gal (0.5%) group showed a trend toward lower levels as compared with the control (Con) group (p = 0.069). Compared with the control (Con) group, the alpha diversity of the bacterial flora of the Shannon index of the Mid_Gal (0.5%) group was significantly higher (p < 0.05). The Simpson index was significantly reduced (p < 0.05), and a PCoA indicated a significant change in the gut microbiota structure among the four groups (p < 0.05). The relative abundance of Blautia and Peptoclostridium in the Low_Gal (0.25%) group was significantly higher as compared with the control (Con) group (p < 0.05). Conclusions: These results indicated that L-β-galactoglucan exhibited a positive effect on improving the palatability and gut microbiota of dogs.