Inclusion Criteria:

1. Confirmed diagnosis of ulcerative colitis (UC) at least 3 months' duration prior to screening
2. Moderately to severely active UC as assessed by the modified Mayo score
3. Documentation of an inadequate response, loss of response, or intolerance to a treatment course of 1 or more of the following standard of care medications: oral 5-aminosalicylic acids, corticosteroids, immunomodulators, anti-tumor necrosis factor (TNF) agents, integrin inhibitors[SA1]
4. Documentation of prior treatment with corticosteroids for ≥ 4 weeks
5. Males and females must agree to follow specific methods of contraception, if applicable

Exclusion Criteria:

1. Current diagnosis of Crohn's disease (CD) or diagnosis of indeterminate colitis (IC), ischemic colitis, or pseudomembranous colitis
2. Current evidence of fulminant colitis, abdominal abscess, toxic megacolon, or bowel perforation
3. History or evidence of any extensive colonic resection, or subtotal or total colectomy
4. Women who are pregnant or breastfeeding
5. Prior exposure to BMS-986165 or a tyrosine kinase 2 (TYK2) inhibitor

Other protocol-defined inclusion/exclusion criteria apply.