Background Simplified regimens for the treatment of human immunodeficiency virus type 1 (HIV-1) infection may increase patient satisfaction and facilitate adherence. Methods In this phase 3, open-label, multicenter, noninferiority trial involving patients who had had plasma HIV-1 RNA levels of less than 50 copies per milliliter for at least 6 months while taking standard oral antiretroviral therapy, we randomly assigned participants (1:1) to either continue their oral therapy or switch to monthly intramuscular injections of long-acting cabotegravir, an HIV-1 integrase strand-transfer inhibitor, and long-acting rilpivirine, a nonnucleoside reverse-transcriptase inhibitor. The primary end point was the percentage of participants with an HIV-1 RNA level of 50 copies per milliliter or higher at week 48, determined with the use of the Food and Drug Administration snapshot algorithm. Results Treatment was initiated in 308 participants per group. At week 48, HIV-1 RNA levels of 50 copies per milliliter or higher were found in 5 participants (1.6%) receiving long-acting therapy and in 3 (1.0%) receiving oral therapy (adjusted difference, 0.6 percentage points; 95% confidence interval [CI], -1.2 to 2.5), a result that met the criterion for noninferiority for the primary end point (noninferiority margin, 6 percentage points). An HIV-1 RNA level of less than 50 copies per milliliter at week 48 was found in 92.5% of participants receiving long-acting therapy and in 95.5% of those receiving oral therapy (adjusted difference, -3.0 percentage points; 95% CI, -6.7 to 0.7), a result that met the criterion for noninferiority for this end point (noninferiority margin, -10 percentage points). Virologic failure was confirmed in 3 participants who received long-acting therapy and 4 participants who received oral therapy. Adverse events were more common in the long-acting-therapy group and included injection-site pain, which occurred in 231 recipients (75%) of long-acting therapy and was mild or moderate in most cases; 1% withdrew because of this event. Serious adverse events were reported in no more than 5% of participants in each group. Conclusions Monthly injections of long-acting cabotegravir and rilpivirine were noninferior to standard oral therapy for maintaining HIV-1 suppression. Injection-related adverse events were common but only infrequently led to medication withdrawal. (Funded by ViiV Healthcare and Janssen; ATLAS ClinicalTrials.gov number, NCT02951052.)