Galla chinensis is rich in polyphenolic compounds and has strong antibacterial ability but has poor palatability and results in fish refusing to eat. Streptococcus iniae is the main pathogen of bacterial diseases of the Asian seabass (Lates calcarifer). Therefore, this study aimed to prepare extracts from the crude herb and modern herbal medicine of G. chinensis to examine the antibacterial abilities of G. chinensis extracts in vitro and their effects on the growth and disease resistance to S. iniae infection in the Asian seabass. The results revealed that all the preparations of G. chinensis, i.e., the crude drug, the modern herbal medicine, and the extracts from the crude drug and modern herbal medicine, have strong antibacterial activity against S. iniae in vitro. The contents of total polyphenols in both G. chinensis extracts were 1.5 times higher than those in the source medicinal materials of G. chinensis. The Asian seabass were fed diets containing 0, 0.5 and 1% of extracts from the crude drug and modern herbal medicine of G. chinensis, respectively, for 14 and 21 days and then challenged with S. iniae. All the G. chinensis extract-added groups had significantly higher disease resistance (p < 0.01) and percent weight gain (p < 0.05) than the control group. There was no significant difference in disease resistance between the groups fed for 14 and 21 days or between the extracts prepared from different material sources ( p > 0.05). If the Asian seabass were fed with G. chinensis extract at doses of 0, 0.5, 1 and 1.5% in feed for 14 days then challenged with S. iniae, the disease resistance of all the extract-added groups was significantly better than that of the control group (p < 0.01), with the protective abilities increased by 25.0 ± 3.8%, 50.0 ± 2.3%, and 56.7 ± 2.3%, respectively. There was no significant difference between the 1% and 1.5% addition groups (p > 0.05). In conclusion, a diet containing G. chinensis extract at a dose of 1% in feed for 14 days is recommended to provide significant protection against S. iniae infection and promote growth in the Asian seabass.