Background: Traditional obesity prevention programs are time-and cost-intensive. Mobile phone technology has been successful in changing behaviors and managing weight; however, to our knowledge, its potential in young children has yet to be examined. Objective: We assessed the effectiveness of a mobile health (mHealth) obesity prevention program on body fat, dietary habits, and physical activity in healthy Swedish children aged 4.5 y. Design: From 2014 to 2015, 315 children were randomly assigned to an intervention or control group. Parents in the intervention group received a 6-mo mHealth program. The primary outcome was fat mass index (FMI), whereas the secondary outcomes were intakes of fruits, vegetables, candy, and sweetened beverages and time spent sedentary and in moderate-to-vigorous physical activity. Composite scores for the primary and secondary outcomes were computed. Results: No statistically significant intervention effect was observed for FMI between the intervention and control group (mean +/- SD: -0.23 +/- 0.56 compared with -0.20 +/- 0.49 kg/m(2)). However, the intervention group increased their mean composite score from baseline to follow-up, whereas the control group did not (+ 0.36 +/- 1.47 compared with -0.06 +/- 1.33 units; P = 0.021). This improvement was more pronounced among the children with an FMI above the median (4.11 kg/m(2)) (P = 0.019). The odds of increasing the composite score for the 6 dietary and physical activity behaviors were 99% higher for the intervention group than the control group (P = 0.008). Conclusions: This mHealth obesity prevention study in preschool-aged children found no difference between the intervention and control group for FMI. However, the intervention group showed a considerably higher postintervention composite score (a secondary outcome) than the control group, especially in children with a higher FMI. Further studies targeting specific obesity classes within preschool-aged children are warranted.