BACKGROUND: College athletes are a group often affected by anxiety. Few interventional studies have been conducted to address the anxiety issues in this population. OBJECTIVE: We conducted a mobile-delivered mindfulness intervention among college athletes to study its feasibility and efficacy in lowering their anxiety level and improving their mindfulness (measured by the Five Facet Mindfulness Questionnaire [FFMQ]). METHODS: In April 2019, we recruited 290 college athletes from a public university in Shanghai, China, and 288 of them were randomized into an intervention group and a control group (closed trial), with the former (n=150) receiving a therapist-guided, smartphone-delivered mindfulness-based intervention and the latter receiving mental health promotion messages (n=138). We offered in-person instructions during the orientation session for the intervention group in a classroom, with the therapist interacting with the participants on the smartphone platform later during the intervention. We used generalized linear modeling and the intent-to-treat approach to compare the 2 groups' outcomes in dispositional anxiety, precompetition anxiety, and anxiety during competition, plus the 5 dimensions of mindfulness (measured by the FFMQ). RESULTS: Our intent-to-treat analysis and generalized linear modeling found no significant difference in dispositional anxiety, precompetition anxiety, or anxiety during competition. Only the "observation" facet of mindfulness measures had a notable difference between the changes experienced by the 2 groups, whereby the intervention group had a net gain of .214 yet fell short of reaching statistical significance (P=.09). Participants who specialized in group sports had a higher level of anxiety (β=.19; SE=.08), a lower level of "nonjudgemental inner experience" in FFMQ (β=-.07; SE=.03), and a lower level of "nonreactivity" (β=-.138; SE=.052) than those specializing in individual sports. CONCLUSIONS: No significant reduction in anxiety was detected in this study. Based on the participant feedback, the time availability for mindfulness practice and session attendance for these student athletes in an elite college could have compromised the intervention's effectiveness. Future interventions among this population could explore a more student-friendly time schedule (eg, avoid final exam time) or attempt to improve cognitive and scholastic outcomes. TRIAL REGISTRATION: Chinese Clinical Trial Registry ChiCTR1900024449; https://www.chictr.org.cn/showproj.html?proj=40865.