Background: Cannabis is one of the most used drugs worldwide. There is no gold standard treatment for cannabis use disorder (CUD). Motivational interviewing (MI) has shown efficacy in some substance use disorders. Therefore, a systematic review was conducted to assess the effectiveness of MI in CUD. Methods: Randomized controlled trials or open-label studies published until September 2019 from 3 different databases (Pubmed, Scopus, and PsycINFO) were included, following the PRISMA guidelines and a predetermined set of criteria for article selection. Meta-analyses were conducted. The end point was determined as month 3, and 4 outcomes were analysed (abstinence rates, reduction in frequency of use, reduction in quantity of use, and reduction in cannabis use disorder symptoms) in 2 populations (adolescents and adults). Results: Forty studies were identified, of which 24 were performed in adults and 16 in adolescents. MI showed efficacy in achieving abstinence in both adults (odds ratio [OR] = 3.84, 95% confidence interval [CI] 2.40–6.16, p < 0.0001) and adolescents (OR = 2.02, 95% CI 1.42–2.89, p < 0.0001). MI showed efficacy in reducing frequency and quantity of use in adults but not in adolescents. Those adults who were in the MI group consumed less joints per day than those in the control group (mean difference = −0.69 joints per day, 95% CI −0.84 to −0.53, p < 0.001), and they consumed on less days per month (mean difference = −3.9 days per month, 95% CI −7.47 to −0.34, p = 0.0317) than those in the control group. Conclusions: MI is an effective intervention to reduce cannabis use and achieve abstinence, especially among adults and patients with no prior history of psychotic disorder. Further investigation is needed to assess the effect on CUD symptoms. MI should be included in guidelines for treating cannabis use disorder as one of the essential psychological interventions.