Despite the importance of primary partners for health, little is known about factors that constrain the ability of couples to work collaboratively towards HIV care and treatment (dyadic coordination). This study examined the interplay of marital infidelity, food insecurity, and couple instability on dyadic coordination and adherence to antiretroviral therapy (ART) in Malawi. In 2016, we conducted 80 in-depth interviews with 25 couples with at least one partner on ART. Couples were recruited at two HIV clinics in the Zomba district when attending clinic appointments. Participants were asked about their relationship history, relationship dynamics (love, trust, conflict), experiences with HIV care and treatment, and how partners were involved. Using an innovative analysis approach, we analyzed the data at the couple-level by examining patterns within and between couples. Three patterns emerged. For some couples, ART led to positive changes in their relationships after men terminated their extramarital partnerships in exchange for love and support. For other couples with power imbalances and ongoing conflict, men's infidelity continued after ART and negatively affected dyadic coordination. Finally, some couples agreed to remain “faithful”, but could not overcome stressors related to food insecurity, which directly impacted their adherence. Couples-based interventions targeting ART adherence should improve relationship quality, while also addressing interpersonal stressors such as marital infidelity and food insecurity. Multi-level interventions that address both dyadic and structural levels may be necessary for couples with severe food insecurity. •Marital infidelity inhibits dyadic coordination around antiretroviral therapy.•Food insecurity, couple conflict, and power dynamics play into this relationship.•Dyadic theory must consider marital stressors among couples living in poverty.