While improvements have been made across the HIV care continuum in South Africa, gaps remain. relationship- focused couples- based approaches may be one avenue to improve HIV- related outcomes for men and women. Prior couples- based studies have been found to improve several HIV care and treatment outcomes in this context, but few have considered viral suppression as the primary outcome. We aimed to compare a couples-based motivational- interviewing intervention delivered to couples to similar content delivered to men and women in couples separately. We will test the efficacy of this approach in a randomised controlled trial. Our goal is to enrol 270 heterosexual couples for this trial, with at least one partner living with HIV. Couples will be randomised into one of two arms, stratified by couplesâ?? HIV status. The intervention arm, Simunye (â??We are oneâ?? in isiZulu), will provide two sessions of motivational information and skills regarding HIV- related behaviours to couples together, along with relationship- focused content and skills. The content is based on Partner Steps (P- steps), a couples- focused adaptation of Life Steps, an evidence- based programme shown to improve adherence and viral suppression. The control group will receive two sessions as individuals, with similar HIV- related information but without relationship- focused content. Participants will be followed up at 6, 12 and 18 months postrandomisation. The baseline questionnaire will include measures of relationship domains such as satisfaction and communication, and measures pertaining to HIV and reproductive health (eg, fertility intentions, HIV knowledge and risk perception, and sexual behaviour), and mental health (eg, depression symptoms). The primary outcome is viral suppression, based on dried blood spots. Secondary outcomes will include other aspects of treatment engagement. We will also examine hypothesised mediators of intervention participation, for example, relationship dynamics. Primary analyses will use a multilevel modelling approach, which will feature planned time- averaged comparisons of postbaseline measurements across the intervention and control groups to test the primary hypothesis. The analysis will account for the dyadic nature of the data, for example, participants nested within couples.