Purpose: Aim of this study was to investigate the association between postdiagnosis body mass index (BMI) and all-cause mortality in colorectal cancer (CRC) survivors in a prospective study and meta-analysis. Methods: We conducted a prospective cohort study on 2,143 CRC survivors in Germany. Participants were recruited to the study on average 4 years after diagnosis, and postdiagnosis BMI was assessed at recruitment using a self-administered questionnaire. CRC survivors were followed up for a mean time of 3.5 years. The association between BMI and all-cause mortality was investigated using multivariable Cox proportional hazards models. Additionally, we performed a meta-analysis of studies on postdiagnosis BMI and all-cause mortality (n = 5, including this study) by applying random-effects models. Results: In the prospective analysis, 349 participants died. BMI was not statistically significantly associated with all-cause mortality. Compared to normal weight survivors, the hazard ratios (HRs) 95 % confidence interval (CI) for all-cause mortality in underweight, overweight and obese survivors were 1.65 (0.79–3.45), 0.80 (0.62–1.03) and 0.84 (0.62–1.14), respectively. In the meta-analysis, individuals with underweight were at increased risk for all-cause mortality HR (95 % CI) 1.72 (1.18–2.49), whereas individuals with overweight had a lower risk HR (95 % CI) 0.79 (0.71–0.88), compared to normal weight subjects. For obesity, the risk of mortality was also reduced with only borderline significance HR (95 % CI) 0.88 (0.77–1.00). Conclusions: While the present study as well as single previously published studies showed that overweight was associated with a non-significant reduced risk for all-cause mortality, our meta-analysis indicated a decreased mortality risk among overweight CRC survivors.