BACKGROUND: International dietary recommendations include guidance on healthy eating and weight management for people who have survived cancer; however dietary interventions are not provided routinely for people living beyond cancer. OBJECTIVES: To assess the effects of dietary interventions for adult cancer survivors on morbidity and mortality, changes in dietary behaviour, body composition, health-related quality of life, and clinical measurements. SEARCH METHODS: We ran searches on 18 September 2019 and searched the Cochrane Central Register of Controlled trials (CENTRAL), in the Cochrane Library; MEDLINE via Ovid; Embase via Ovid; the Allied and Complementary Medicine Database (AMED); the Cumulative Index to Nursing and Allied Health Literature (CINAHL); and the Database of Abstracts of Reviews of Effects (DARE). We searched other resources including reference lists of retrieved articles, other reviews on the topic, the International Trials Registry for ongoing trials, metaRegister, Physicians Data Query, and appropriate websites for ongoing trials. We searched conference abstracts and WorldCat for dissertations. SELECTION CRITERIA: We included randomised controlled trials (RCTs) that recruited people following a cancer diagnosis. The intervention was any dietary advice provided by any method including group sessions, telephone instruction, written materials, or a web-based approach. We included comparisons that could be usual care or written information, and outcomes measured included overall survival, morbidities, secondary malignancies, dietary changes, anthropometry, quality of life (QoL), and biochemistry. DATA COLLECTION AND ANALYSIS: We used standard Cochrane methodological procedures. Two people independently assessed titles and full-text articles, extracted data, and assessed risk of bias. For analysis, we used a random-effects statistical model for all meta-analyses, and the GRADE approach to rate the certainty of evidence, considering limitations, indirectness, inconsistencies, imprecision, and bias. MAIN RESULTS: We included 25 RCTs involving 7259 participants including 977 (13.5%) men and 6282 (86.5%) women. Mean age reported ranged from 52.6 to 71 years, and range of age of included participants was 23 to 85 years. The trials reported 27 comparisons and included participants who had survived breast cancer (17 trials), colorectal cancer (2 trials), gynaecological cancer (1 trial), and cancer at mixed sites (5 trials). For overall survival, dietary intervention and control groups showed little or no difference in risk of mortality (hazard ratio (HR) 0.98, 95% confidence interval (CI) 0.77 to 1.23; 1 study; 3107 participants; low-certainty evidence). For secondary malignancies, dietary interventions versus control trials reported little or no difference (risk ratio (RR) 0.99, 95% CI 0.84 to 1.15; 1 study; 3107 participants; low-certainty evidence). Co-morbidities were not measured in any included trials. Subsequent outcomes reported after 12 months found that dietary interventions versus control probably make little or no difference in energy intake at 12 months (mean difference (MD) -59.13 kcal, 95% CI -159.05 to 37.79; 5 studies; 3283 participants; moderate-certainty evidence). Dietary interventions versus control probably led to slight increases in fruit and vegetable servings (MD 0.41 servings, 95% CI 0.10 to 0.71; 5 studies; 834 participants; moderate-certainty evidence); mixed results for fibre intake overall (MD 5.12 g, 95% CI 0.66 to 10.9; 2 studies; 3127 participants; very low-certainty evidence); and likely improvement in Diet Quality Index (MD 3.46, 95% CI 1.54 to 5.38; 747 participants; moderate-certainty evidence). For anthropometry, dietary intervention versus control probably led to a slightly decreased body mass index (BMI) (MD -0.79 kg/m², 95% CI -1.50 to -0.07; 4 studies; 777 participants; moderate-certainty evidence). Dietary interventions versus control probably had little or no effect on waist-to-hip ratio (MD -0.01, 95% CI -0.04 to 0.02; 2 studies; 106 participants; low-certainty evidence). For QoL, there were mixed results; several different quality assessment tools were used and evidence was of low to very low-certainty. No adverse events were reported in any of the included studies. AUTHORS' CONCLUSIONS: Evidence demonstrated little effects of dietary interventions on overall mortality and secondary cancers. For comorbidities, no evidence was identified. For nutritional outcomes, there was probably little or no effect on energy intake, although probably a slight increase in fruit and vegetable intake and Diet Quality Index. Results were mixed for fibre. For anthropometry, there was probably a slight decrease in body mass index (BMI) but probably little or no effect on waist-to-hip ratio. For QoL, results were highly varied. Additional high-quality research is needed to examine the effects of dietary interventions for different cancer sites, and to evaluate important outcomes including comorbidities and body composition. Evidence on new technologies used to deliver dietary interventions was limited.