A Study to Learn How Safe ACT-777991 is and How Well it Works in Adults With Non-segmental Vitiligo

The purpose of this clinical trial is to learn how well ACT-777991 works, how safe it is and how well it is tolerated by adults with non-segmental vitiligo. The main question this clinical trial aims to answer is: • Can ACT-777991 help return color to the skin of the face of adults with non-segmental vitiligo? Researchers will compare ACT-777991 to placebo (a look-alike inactive treatment that contains no medicine) to see if ACT-777991 works to treat non-segmental vitiligo. Trial participants will: * Take the trial intervention (either ACT-777991 or placebo) daily for 24 weeks. * Visit the clinic 7 times for check-up and tests.

Inclusion Criteria: * Clinical diagnosis of either active or stable non segmental vitiligo for at least 3 months prior to Screening and meet all the following criteria: * F-VASI score ≥ 0.3 based on BICR at Screening. * T-VASI score ≥ 5 based on investigator assessment at Screening and Randomization. * Total body surface area (BSA) involvement, including the face, ≤ 50% based on investigator assessment at Screening and Randomization. * Participants must agree not to use therapeutic agents and procedures to treat vitiligo from Screening until Participant Last Visit. Applicable only to EU: In addition to the above, participants must not have discontinued, reduced, or modified a vitiligo treatment/ procedure for the purpose of meeting trial eligibility criteria or a wash out requirement. Exclusion Criteria: * Clinical diagnosis of other forms of vitiligo (e.g., segmental) or other hypo- or depigmentation disorders (e.g., piebaldism, leukoderma, Vogt-Koyanagi-Harada disease, malignancy-induced hypopigmentation). * Any autoimmune disease, except adequately treated thyroid disease. * History of systemic immunotherapy treatment, including JAK inhibitors, for any inflammatory disease in the 12 months prior to Randomization. * History of topical JAK inhibitors for any inflammatory disease in the 6 weeks prior to Screening. * Use of laser or light-based treatment (phototherapy), including tanning beds, in the 8 weeks prior to Screening. * eGFR \< 90 mL/min/1.73 m\^2, defined by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation, at Screening.