Developmental dysplasia of the hip (DDH) can cause pain and gait disturbances but may be prevented with appropriate care. This study investigated parental knowledge and practices regarding DDH prevention in Japan and explored associated background factors. A web-based survey targeting primary caregivers of infants under 1 year was conducted from August to October 2024. The questionnaire included questions about parental and infant characteristics, DDH-related knowledge, and prevention practices. Regarding DDH-related knowledge, we developed a 10-item scale. A total of 1262 participants were included in the analyses. The correct response rate for the 10 DDH knowledge items was 41.2% overall, and fathers scored significantly lower (33.2%) than mothers (46.2%). Parents who were female, higher educational level, born in winter, and home visits were associated with greater knowledge of DDH. In terms of practice, 13.5% of the parents did not adopt the recommended way of holding their infants, a technique that helps prevent DDH. In addition, 21.9% reported using swaddling, a known risk factor for DDH. Among the 537 participants whose infants had a positional preference, 43.2% did not take corrective action to address it. Less than half of the Japanese parents demonstrated adequate knowledge of DDH, and more than 10% engaged in caregiving practices that could increase the risk of DDH. The education of parents regarding appropriate infant-holding techniques may be effective in preventing DDH. Enhancing health education regarding DDH risk factors during home visits and infant health checkups can improve parental knowledge and practice.