Although several observational studies have demonstrated an association between vitamin K status and bone mineral density (BMD) in postmenopausal women, no placebo-controlled intervention trials of the effect of vitamin K₁ supplementation on bone loss have been reported thus far. In the trial presented here we have investigated the potential complementary effect of vitamin K₁ (1 mg/day) and a mineral + vitamin D supplement (8 µg/day) on postmenopausal bone loss. The design of our study was a randomized, double-blind, placebo-controlled intervention study; 181 healthy postmenopausal women between 50 and 60 years old were recruited, 155 of whom completed the study. During the 3-year treatment period, participants received a daily supplement containing either placebo, or calcium, magnesium, zinc, and vitamin D (MD group), or the same formulation with additional vitamin K₁ (MDK group). The main outcome was the change in BMD of the femoral neck and lumbar spine after 3 years, as measured by DXA. The group receiving the supplement containing additional vitamin K₁ showed reduced bone loss of the femoral neck: after 3 years the difference between the MDK and the placebo group was 1.7% (95% Cl: 0.35–3.44) and that between the MDK and MD group was 1.3% (95% Cl: 0.10–3.41). No significant differences were observed among the three groups with respect to change of BMD at the site of the lumbar spine. If co-administered with minerals and vitamin D, vitamin K₁ may substantially contribute to reducing postmenopausal bone loss at the site of the femoral neck.