Background/Objectives: Incidence of malnutrition varies greatly among gastrointestinal (GI) cancer patients and has a major impact on prognosis. We performed a meta-analysis to identify risk factors for malnutrition risk, malnutrition diagnosis, and cachexia in patients with GI cancer. Methods: A systematic search was performed on 31 October 2025 on the PubMed (Medline), Embase, and Cochrane Library databases. Eligible studies reported on risk factors for malnutrition risk, malnutrition diagnosis, malnutrition-related complication risk and cachexia in adult patients with GI cancer. Articles on neuroendocrine tumours, primary cancer outside the GI tract, and the paediatric population were excluded. The random-effects model yielded the pooled odds ratios (ORs) and 95% confidence intervals (CIs) for the investigated risk factors. Results: A total of 37,624 records were identified. Data from 262,525 patients from 578 articles were included in the analysis. Older age (≥65) was associated with higher odds for malnutrition risk across all GI cancers. In gastric cancer, males had a lower odds for malnutrition risk (OR 0.84; 95% CI 0.75–0.95); however, the sex difference across other cancer types was heterogeneous, and mostly not significant. Tumour location influenced the odds for malnutrition-related complication risk in pancreatic ductal adenocarcinoma (head vs. body/tail—OR 1.48; 95% CI 0.98–2.23) and colorectal cancer (colon vs. rectal—OR 1.39; 95% CI 1.07–1.81; right-sided vs. left-sided—OR 1.54; 95% CI 1.34–1.77). Increased C-reactive protein alone indicated higher odds for malnutrition risk at baseline. Conclusions: Inflammatory biomarkers and tumour characteristics may indicate malnutrition risk in GI cancer at baseline. There is a great need for standardised and harmonised approaches in nutritional status assessment in GI cancer.