With the global rise in obesity and the metabolic syndrome, double diabetes is increasingly prevalent in patients with type 1 diabetes. This review investigated the effects of diet on weight management and metabolic outcomes in patients with double diabetes. MEDLINE, CENTRAL, EMBASE, PsycINFO, CINAHL, ERIC, and Web of Science databases were searched through September 2020. Population- or individual-level dietary interventions, and observational studies investigating dietary patterns in adults with type 1 diabetes and overweight or obesity, were eligible for inclusion. The quality of studies was assessed. Four eligible studies were included in this review, comprising two randomized controlled trials, one pretest-posttest study, and one cross-sectional study. Study populations included between 10 and 1040 participants. Dietary interventions included the Mediterranean diet, low-fat diet, intermittent fasting, continuous energy restriction, and a combination of fasting and a standardized low-calorie diabetic diet (LCD). Significant weight loss was observed within groups for low-fat diet, Mediterranean diet, fasting, LCD with fasting, intermittent fasting, or continuous energy restriction, but there were no between-group differences. Weight maintenance was only achieved in interventions where fasting or intermittent fasting were present. Dietary interventions in published data failed to demonstrate effects on metabolic syndrome. Larger sample, high-quality trials conducted over longer periods are urgently required to determine the efficacy of diet for weight management and improving metabolic outcomes in individuals with double diabetes. This would provide much needed evidence-based guidance for dietary interventions, which are well known to be the cornerstone of clinical care.