Objective: The primary objective of this study was to evaluate the feasibility and preliminary efficacy of a school-based physical fitness intervention (Fit4Fun) on the physical fitness and physical activity (PA) levels of primary school children. Methods: A group-randomized controlled trial with a 3-month wait-list control group was conducted in two primary schools in the Hunter Region, NSW, Australia. Participants ("n"?=?48 students; mean age?=?10.9 years?[plus or minus]?0.7) were randomized by the schools into the Fit4Fun intervention ("n"?=?32 students) or the control ("n"?=?17) conditions. Fit4Fun was an 8-week programme that included: 8?[times]?60 [minutes] health and physical education (HPE) lessons, a break-time activity programme (recess and lunch) and a home fitness programme. The control group participated in their usual weekly 60 [minutes] HPE lessons. Assessments were taken at baseline and post-intervention (8-week) to determine changes in health-related fitness (HRF) levels, PA and attitudes towards HRF testing. Objectively measured PA (mean steps/day) was assessed using 7 days of pedometery and HRF was assessed using a battery of tests including: seven-stage sit-up test, push-up test, basketball throw, wall squat, sit and reach, shoulder stretch, 20 [minute] shuttle test, and height and weight measurements. A questionnaire was also administered to assess perceptions of physical fitness and physical fitness testing and changes in attitudes to fitness testing. Intervention effects were assessed using analysis of covariance and Cohen's "d" effect sizes are reported. Results: Children in the intervention group improved in all HRF measures with significant group [multiplied by] time effects ("p"?<?0.05) observed in the seven-stage sit-up test ("d"?=?0.9), the sit and reach tests (right leg "d"?=?1.0, left leg "d"?=?0.9, both legs "d"?=?1.1) and the wall squat tests (right leg "d"?=?0.9, left leg "d"?=?0.6). No significant group [multiplied by] time effect was found in the beep test, basketball throw, PA measure or psychological measures. The control group did not display significant within-group effects for any measure. Conclusions: Results indicate that a multi-component HRF intervention for primary school children that targeted the three areas of a health-promoting school and incorporated social support for participation in physical fitness activities was feasible and efficacious in improving muscular fitness and flexibility levels of children.