We investigated the effects of d-β-hydroxybutyric acid (D-BHB) on sleep quality in healthy Japanese adults. In this randomized, placebo-controlled, double-blind, parallel-group study, each group comprised 30 healthy Japanese adults. Participants received 1.5 g of D-BHB (low D-BHB group), 2.9 g of D-BHB (high D-BHB group), or a placebo beverage (placebo group) for 14 days. Before and after the intervention, the Oguri-Shirakawa-Azumi sleep inventory, middle-aged and aged version (OSA-MA), and sleep state test were conducted. After 14 days, compared to the placebo group, the OSA-MA scores for "Sleepiness on rising" and "Frequent dreaming" were significantly higher in both the low and high D-BHB groups. Additionally, the score for "Initiation and maintenance of sleep" was significantly higher in the low D-BHB group, and the score for "Refreshing on rising" was significantly higher in the high D-BHB group. We found that D-BHB can improve sleep quality in healthy Japanese adults.