Vitamin D is crucial for calcium and phosphorous homeostasis and for bone health. Evidence also has linked vitamin D deficiency with impaired immune function. Studies conducted in populations that reside close to the equator show that vitamin D insufficiency is prevalent despite abundant UVB. Response to oral vitamin D supplementation in this population is not known. The aim of this study was to determine effect of vitamin D supplementation on maternal and infant plasma 25(OH)D concentrations, and on markers of bone turnover, infant growth, gross motor development and morbidity. A randomized, placebo-controlled, double-blind trial (NCT02210884) was conducted in Sidama Zone (7°N, 38°E), Southern Ethiopia. Lactating women (n=126) enrolled within two weeks postpartum were randomized to vitamin D3 (15,000 IU/week) or placebo for 12 months. Plasma 25(OH)D, bone-specific alkaline phosphatase (BAP), osteocalcin (OC) and C-telopeptide for type 1 collagen (CTX) were determined. Skin reflectance, dietary intake, and individual UVB exposure were also measured. Moreover, infant health and growth outcomes were also assessed. Median (IQR) 25(OH)D concentrations were not significantly different between vitamin D and control groups at baseline [45 (39, 56) nmol/L vs 47 (37, 57) nmol/L, p = 0.697]. 25(OH)D concentrations were significantly higher in the vitamin D group at end-line [109 (93, 121) nmol/L vs 63 (49, 81) nmol/L p < 0.0001]. All women in the vitamin D group were vitamin D sufficient (> 50 nmol/L) by the end of the study. Furthermore, 95% had attained 25(OH)D concentrations > 75 nmol/L compared to 39% in the control group. End-line infant 25(OH)D concentrations were not significantly different between groups. Maternal BAP and OC concentrations significantly increased from two weeks post-partum to end-line in both groups ( p < 0.001). No significant differences by treatment were seen for all bone turnover markers, infant growth, and health outcomes ( p = 0.974) . Weekly vitamin D supplementation safely eliminated vitamin D insufficiency in the study population. Our results suggest that supplementation can be used to improve vitamin D status in populations with limited sun exposure and low dietary vitamin D and calcium intake. Intervention did not affect bone turnover and infant health outcomes.