Objectives: Patient-reported cognitive complaint (CI) is poorly described in older adults with cancer. The purpose of this study was to quantify the prevalence and examine the associations of patient-reported CI in older adults with gastrointestinal (GI) malignancies at diagnosis. Materials and Methods: Using participants ≥60 years with GI malignancies from the Cancer & Aging Resilience Evaluation (CARE) Registry that underwent a Geriatric Assessment (GA), we examined CI using the Patient-Reported Outcomes Measurement Information System (PROMIS®) Short Form 4a Cognitive Function. Cognition scores were dichotomized into normal (scores of 15–20) and impaired (4–14), and bivariate and multivariate analyses were used to examine associations. Results: A total of 264 participants with GI malignancy were included, mean age of 70.0 ± 7.1, 55.7% male, pancreatic cancer was the most common cancer (24.2%) and majority were stage III/IV (68.2%). 29.3% of participants endorsed CI. CI was not associated with demographic and clinical domains, but was associated with many GA impairments including instrumental Activities of Daily Living (iADL) impairment (adjusted odds ratio [aOR] 6.0, 95% confidence interval 3.0–11.8), falls (aOR 2.7, 1.4–5.4), anxiety (aOR 10.3, 5.2–20.4), and depression (aOR 10.2, 5.2–20.4). CI was also associated with reduced global mental (aOR 18.7, 8.1–42.2) and physical (aOR 4.7, 2.4–8.9) quality of life, and prior hospitalizations (aOR 2.4, 1.2–4.8). Conclusion: We found a high prevalence of patient-reported CI in older adults with GI malignancies that was associated with increased GA impairments, reduced health-related quality of life, and increased healthcare utilization.