Background Child overweight and obesity has increased globally, and can be associated with short‐ and long‐term health consequences. Objectives To assess the effects of diet, physical activity, and behavioural interventions for the treatment of overweight or obesity in preschool children up to the age of 6 years. Search methods We performed a systematic literature search in the databases Cochrane Library, MEDLINE, EMBASE, PsycINFO, CINAHL, and LILACS, as well as in the trial registers ClinicalTrials.gov and ICTRP Search Portal. We also checked references of identified trials and systematic reviews. We applied no language restrictions. The date of the last search was March 2015 for all databases. Selection criteria We selected randomised controlled trials (RCTs) of diet, physical activity, and behavioural interventions for treating overweight or obesity in preschool children aged 0 to 6 years. Data collection and analysis Two review authors independently assessed risk of bias, evaluated the overall quality of the evidence using the GRADE instrument, and extracted data following the Cochrane Handbook for Systematic Reviews of Interventions. We contacted trial authors for additional information. Main results We included 7 RCTs with a total of 923 participants: 529 randomised to an intervention and 394 to a comparator. The number of participants per trial ranged from 18 to 475. Six trials were parallel RCTs, and one was a cluster RCT. Two trials were three‐arm trials, each comparing two interventions with a control group. The interventions and comparators in the trials varied. We categorised the comparisons into two groups: multicomponent interventions and dietary interventions. The overall quality of the evidence was low or very low, and six trials had a high risk of bias on individual 'Risk of bias' criteria. The children in the included trials were followed up for between six months and three years. In trials comparing a multicomponent intervention with usual care, enhanced usual care, or information control, we found a greater reduction in body mass index (BMI) z score in the intervention groups at the end of the intervention (6 to 12 months): mean difference (MD) ‐0.3 units (95% confidence interval (CI) ‐0.4 to ‐0.2); P < 0.00001; 210 participants; 4 trials; low‐quality evidence, at 12 to 18 months' follow‐up: MD ‐0.4 units (95% CI ‐0.6 to ‐0.2); P = 0.0001; 202 participants; 4 trials; low‐quality evidence, and at 2 years' follow‐up: MD ‐0.3 units (95% CI ‐0.4 to ‐0.1); 96 participants; 1 trial; low‐quality evidence. One trial stated that no adverse events were reported; the other trials did not report on adverse events. Three trials reported health‐related quality of life and found improvements in some, but not all, aspects. Other outcomes, such as behaviour change and parent‐child relationship, were inconsistently measured. One three‐arm trial of very low‐quality evidence comparing two types of diet with control found that both the dairy‐rich diet (BMI z score change MD ‐0.1 units (95% CI ‐0.11 to ‐0.09); P < 0.0001; 59 participants) and energy‐restricted diet (BMI z score change MD ‐0.1 units (95% CI ‐0.11 to ‐0.09); P < 0.0001; 57 participants) resulted in greater reduction in BMI than the comparator at the end of the intervention period, but only the dairy‐rich diet maintained this at 36 months' follow‐up (BMI z score change in MD ‐0.7 units (95% CI ‐0.71 to ‐0.69); P < 0.0001; 52 participants). The energy‐restricted diet had a worse BMI outcome than control at this follow‐up (BMI z score change MD 0.1 units (95% CI 0.09 to 0.11); P < 0.0001; 47 participants). There was no substantial difference in mean daily energy expenditure between groups. Health‐related quality of life, adverse effects, participant views, and parenting were not measured. No trial reported on all‐cause mortality, morbidity, or socioeconomic effects. All results should be interpreted cautiously due to their low quality and heterogeneous interventions and comparators. Authors' conclusions Muticomponent interventions appear to be an effective treatment option for overweight or obese preschool children up to the age of 6 years. However, the current evidence is limited, and most trials had a high risk of bias. Most trials did not measure adverse events. We have identified four ongoing trials that we will include in future updates of this review. The role of dietary interventions is more equivocal, with one trial suggesting that dairy interventions may be effective in the longer term, but not energy‐restricted diets. This trial also had a high risk of bias.