Flaxseed consumption may be inversely associated with obesity; however, findings of available randomized controlled trials (RCTs) are conflicting. The present study aimed to systematically review and analyse RCTs assessing the effects of flaxseed consumption on body weight and body composition. PubMed, Medline via Ovid, SCOPUS, EMBASE and ISI Web of Sciences databases were searched up to November 2016. Mean changes in body composition indices including body weight, body mass index (BMI) and waist circumference were extracted. Effect sizes were expressed as weighted mean difference (WMD) and 95% confidence intervals (CI). Heterogeneity between studies was assessed with the I2 test. Publication bias and subgroup analyses were also performed. The quality of articles was assessed via the Jadad scale. A total of 45 RCTs were included. Meta‐analyses suggested a significant reduction in body weight (WMD: −0.99 kg, 95% CI: −1.67, −0.31, p = 0.004), BMI (WMD: −0.30 kg m−2, 95% CI: −0.53, −0.08, p = 0.008) and waist circumference (WMD: −0.80 cm, 95% CI: −1.40, −0.20, p = 0.008) following flaxseed supplementation. Subgroup analyses showed that using whole flaxseed in doses ≥30 g d−1, longer‐term interventions (≥12 weeks) and studies including participants with higher BMI (≥ 27 kg m−2) had positive effects on body composition. Whole flaxseed is a good choice for weight management particularly for weight reduction in overweight and obese participants.