The pathogenesis of inflammatory bowel disease (IBD) is linked to an intricate association of environmental, microbial, and host-related factors. This study examined the potential effects of dietary addition of two preparations from onion, one comprising quercetin aglycone alone (Q: 0.15% polyphenols, quercetin aglycone:quercetin monoglycosides, 98:2) and another comprising quercetin aglycone with monoglycosides (Q+MQ: 0.15% total polyphenols, quercetin aglycone:quercetin monoglycosides, 69:31), on dextran sodium sulphate- (DSS-) induced colitis in mice. The results revealed a significant decrease in the body weight gain of the mice with DSS-induced colitis, which was counteracted by the dietary Q or Q+MQ supplementation. Meanwhile, the oxidative stress indicated by myeloperoxidase (MPO), reduced glutathione (GSH), malondialdehyde (MDA), and serum nitrate (NO) concentrations was higher in mice with DSS-induced colitis than in the control group mice, but dietary Q or Q+MQ supplementation counteracted this trend. The colitis mice demonstrated reduced Chao1, angiotensin-converting enzyme (ACE), and Shannon indices and an increased Simpson index, but the colitis mice receiving dietary Q or Q+MQ exhibited higher Chao1, ACE, and Shannon indices and a reduced Simpson index. In conclusion, this research showed that even at a low dose, dietary Q or Q+MQ supplementation counteracts DSS-induced colitis in mice, indicating that Q or Q+MQ may be used as an adjuvant therapy for IBD patients.