Objectives: This study aimed to determine the effects of participatory workplace improvement (PWI)-based provision of ergonomic training and ergonomic action checklists (ACLs) to on-site managers on workplace improvement activities for low back pain (LBP). Methods: A randomized controlled trial (RCT) was conducted at a manufacturing company in Japan. Teams entered in the study were randomly assigned to a control and an intervention group. A total of three interventional training sessions on methods of ergonomics were provided to on-site managers in the intervention group, with 1-month intervals between sessions. Ergonomic ACLs were provided at the same time. After completion of the training sessions, each team then provided a report of improvements each month for the next 10 months. Two people in charge of safety and health chose two major objectives of the implemented activities from the five categories. The reported number of improvements was analyzed using a Poisson regression model. Results: In the intervention group, although the incident rate ratio (IRR) of PWIs in countermeasures for the LBP category was significantly elevated after the training sessions, the IRR of improvements decreased over time during the 10-month follow-up period. No significant difference was observed in the IRR of total PWIs in either the control or intervention group. Conclusions: PWI-based provision of ergonomic training sessions and ergonomics ACLs to on-site managers was shown to be effective for workplace improvement activities targeted at LBP. However, because the effects decrease over time, efforts should be made to maintain the effects through regular interventions.