Background Observational studies suggest that vitamin D deficiency among people living with HIV is associated with a greater risk of disease progression and death. Low levels of vitamin D in pregnancy are also associated with poor fetal and infant growth. Therefore, vitamin D supplementation may improve clinical outcomes for pregnant women living with HIV and improve fetal and postnatal growth for their infants. Methods and findings We conducted a randomized, triple-blind, placebo-controlled trial of vitamin D₃ supplementation among pregnant and lactating women living with HIV in Dar es Salaam, Tanzania (ClinicalTrials.gov NCT02305927). Participants were randomized with 1:1 allocation stratified by study clinic to receive either daily 3,000 IU vitamin D₃ supplements or matching placebo supplements from the second trimester of pregnancy (12–27 weeks) until 1 year postpartum. The primary outcomes were (i) maternal HIV progression or death, (ii) small-for-gestational-age (SGA) live births (<10th percentile), and (iii) infant stunting at 1 year of age (length-for-age z-score < −2). We also examined the effect of vitamin D₃ supplementation on secondary maternal and infant health outcomes, maternal and infant serum 25-hydroxyvitamin D (25[OH]D) concentrations, and maternal hypercalcemia. An intent-to-treat analysis was used as the primary analytic approach. We enrolled 2,300 pregnant women between June 15, 2015, and April 17, 2018, and follow-up of mothers and infants was completed on October 20, 2019. There were 1,148 pregnant women randomly assigned to the vitamin D₃ group, and 1,152 to the placebo group. The proportion of mothers lost to follow-up at 1 year postpartum was 6.6% in the vitamin D₃ group (83 of 1,148) and 6.6% in the placebo group (76 of 1,152). The proportion of children lost to follow-up at 1 year of age was 5.5% in the vitamin D₃ group (59 of 1,074 live births) and 5.2% in the placebo group (57 of 1,093 live births). There was no difference in the risk of maternal HIV progression or death, with 166 events during 1,461 person-years of follow-up in the vitamin D₃ group and 141 events during 1,469 person-years of follow-up in the placebo group (hazard ratio 1.21, 95% CI 0.97 to 1.52, p = 0.09). There was no difference in the risk of SGA birth between the vitamin D₃ (229 SGA births among 1,070 live births) and placebo groups (236 SGA births among 1,091 live births) (relative risk 1.03, 95% CI 0.87 to 1.22, p = 0.70). There was also no difference in the risk of infant stunting at 1 year of age between the vitamin D₃ (407 events among 867 infants) and placebo groups (413 events among 873 infants) (relative risk 1.00, 95% CI 0.92 to 1.10, p = 0.95). In terms of adverse events, no cases of maternal hypercalcemia were identified. One hypersensitivity reaction to the trial supplements occurred for a pregnant woman in the placebo group. A limitation of our study is that our findings may not be generalizable to HIV-negative pregnant women or contexts where severe vitamin D deficiency is prevalent. Conclusions The trial findings do not support routine vitamin D supplementation for pregnant and lactating women living with HIV in Tanzania. Trial registration ClinicalTrials.gov Identifier: NCT02305927.