Inclusion Criteria:

• Phase 2: Is human immunodeficiency virus type 1 (HIV-1) positive with Plasma HIV-1 ribonucleic acid (RNA) ≥500 and ≤100,000 copies/mL.
• Phase 3: Is HIV-1 positive with Plasma HIV-1 RNA ≥500 copies/mL.
• Phase 2: Has cluster of differentiation 4-positive (CD4+) T-cell count ≥200 cells/mm^3.
• Is naïve to antiretroviral therapy (ART), defined as having received no prior therapy with any antiretroviral agent following a diagnosis of HIV 1 infection.

Exclusion Criteria:

• Has human immunodeficiency virus type 2 (HIV-2) infection.
• Has a diagnosis of an active acquired immune deficiency syndrome (AIDS)-defining opportunistic infection.
• Has active hepatitis C virus (HCV) or active hepatitis B virus (HBV) infection.
• Has a history of malignancy ≤5 years prior to providing documented informed consent except for adequately treated basal cell or squamous cell skin cancer, in situ cervical or in situ anal cancer, or cutaneous Kaposi's sarcoma.
• Has prior exposure to islatravir (ISL) or ulonivirine (ULO) for any duration any time prior to Day 1.