PURPOSE: Frailty can affect cancer treatment decisions and outcomes. Depression, fatigue, and cognitive impairment often experienced by patients with cancer are expected to be associated with frailty. We aimed to identify frailty and its related factors in older adults with cancer undergoing chemotherapy. METHODS: We conducted a cross-sectional descriptive study in a Korean urban tertiary hospital. A structured self-report questionnaire was used including frailty and its related variables and measured heart rate variability (HRV). Multinomial logistic regression was performed to identify the factors related to frailty. RESULTS: Data from 124 out of 136 patients with stomach, colorectal, and lung cancers were analyzed. Approximately 90% of the participants were in the pre-frail and frail groups. There were significant differences among the three groups in terms of depression (p = 0.006) and HRV (p = 0.041). The factor associated with higher frailty levels was depression across groups (pre-frail odds ratio (OR): 1.31, 95% CI: 1.14-1.51; frail OR: 1.29, 95% CI: 1.11-1.49). However, only deficits observed or commented on by others were retained as factors significantly associated with higher frailty in the pre-frail group (OR: 2.40, 95% CI: 1.40-4.13). In contrast, increased HRV (OR: 0.35, 95% CI: 0.13-0.95) was associated with lower frailty levels in the frail group. CONCLUSION: It is important to classify older adult patients with cancer into pre-frail and frail groups so that interventions can be provided on time. Understanding the characteristics associated with frailty in older adult patients with cancer can positively affect their health-related quality of life and treatment outcomes.