Black men have a higher risk of prostate cancer compared with White men, but Black adults are underrepresented in online content about prostate cancer. Across racial groups, the internet is a popular source of health information; Black adults are more likely to trust online health information, yet have more medical mistrust than White adults. To evaluate the association between racial representation in online content about prostate cancer and trust in the content and identify factors that influence trust. A randomized clinical trial was conducted from August 18, 2021, to January 7, 2022, consisting of a 1-time online survey. Participants included US men and women aged 40 years and older. Data were analyzed from January 2022 to June 2023. Participants were randomized to watch the same video script about either prostate cancer screening or clinical trials presented by 1 of 4 speakers: a Black physician, a Black patient, a White physician, or a White patient, followed by a questionnaire. The primary outcome was a published scale for trust in the information. χ2 tests and multivariable logistic regression were used to compare trust according to the video's speaker and topic. Among 2904 participants, 1801 (62%) were men, and the median (IQR) age was 59 (47-69) years. Among 1703 Black adults, a greater proportion had high trust in videos with Black speakers vs White speakers (72.7% vs 64.3%; adjusted odds ratio [aOR], 1.62; 95% CI, 1.28-2.05; P < .001); less trust with patient vs physician presenter (64.6% vs 72.5%; aOR, 0.63; 95% CI, 0.49-0.80; P < .001) and about clinical trials vs screening (66.3% vs 70.7%; aOR, 0.78; 95% CI, 0.62-0.99; P = .04). Among White adults, a lower proportion had high trust in videos featuring a patient vs physician (72.0% vs 78.6%; aOR, 0.71; 95% CI, 0.54-0.95; P = .02) and clinical trials vs screening (71.4% vs 79.1%; aOR, 0.57; 95% CI, 0.42-0.76; P < .001), but no difference for Black vs White presenters (76.8% vs 73.7%; aOR, 1.11; 95% CI, 0.83-1.48; P = .49). In this randomized clinical trial, prostate cancer information was considered more trustworthy when delivered by a physician, but racial concordance was significantly associated with trust only among Black adults. These results highlight the importance of physician participation and increasing racial diversity in public dissemination of health information and an ongoing need for public education about clinical trials. ClinicalTrials.gov Identifier: NCT05886751.