Background: Cancer clinical trial (CCT) participants are at high risk for experiencing adverse effects from financial toxicity, yet this remains understudied in the CCT population. We sought to describe associations among patient-reported financial toxicity (financial burden [FB] and trial cost concerns), physical and psychological symptoms, illness perceptions, and communication confidence in CCT participants. Methods: From 7/2015-7/2017, we prospectively enrolled CCT participants who expressed interest in financial assistance and a group of patients matched by age, sex, cancer type, specific trial, and trial phase. We assessed FB (burdened by costs of cancer care), trial cost concerns (worried about affording medical costs of a CCT), physical (Edmonton Symptom Assessment Scale [ESAS]) and psychological (Patient Health Questionnaire-4 [PHQ-4]) symptoms, illness perceptions (Brief Illness Perception Questionnaire [BIPQ]), and communication confidence (Perceived Efficacy in Patient-Physician Interactions [PEPPI]). We used regression models to explore sociodemographic associations with FB and trial cost concerns, and to examine associations of FB and trial cost concerns with patients’ symptom burden, illness perceptions, and communication confidence, adjusting for age, sex, race, performance status, marital status, and metastatic status. Results: Of 198 patients enrolled, 112 (56.6%) reported FB and 82 (41.4%) had trial cost concerns. Patients with FB were younger (OR 0.96, 95% CI 0.94-0.98) and had lower incomes ( < $100,000, OR 4.61, 95% CI 2.35-9.01). Patients reporting trial cost concerns also had lower incomes ( < $100,000, OR 2.78, 95% CI 1.45-5.29). On adjusted analyses, patients with FB had higher ESAS total (OR 1.03, 95% CI 1.02-1.05), ESAS physical (OR 1.04, 95% CI 1.02-1.07), PHQ-4 depression (OR 1.54, 95% CI 1.22-1.94), and PHQ-4 anxiety (OR 1.30, 95% CI 1.08-1.55) scores, as well as more negative illness perceptions (OR 1.04, 95% CI 1.01-1.07), but no significant difference in communication confidence (OR 0.98, 95% CI 0.93-1.05). Patients reporting trial cost concerns had higher ESAS total (OR 1.03, 95% CI 1.01-1.05), ESAS physical (OR 1.04, 95% CI 1.01-1.06), PHQ-4 depression (OR 1.35, 95% CI 1.10-1.65), and PHQ-4 anxiety (OR 1.27, 95% CI 1.07-1.51) scores, as well as more negative illness perceptions (OR 1.06, 95% CI 1.03-1.10), and lower communication confidence (OR 0.93, 95% CI 0.87-0.99). Conclusions: In this study of CCT participants, younger patients with lower incomes were most vulnerable to financial toxicity. Financial toxicity was associated with greater symptom burden, more negative illness perceptions, and lower communication confidence, which underscores the importance of addressing these issues when seeking to alleviate the adverse effects of financial toxicity in CCT participants.