Objective: Our objectives were to measure reporting differences between sexual behavior data from daily diary and retrospective estimates and to assess the utility of using smartphones to collect sexual behavior data from a college student population. Participants: Eighty-six participants (68 women, 18 men) completed the study. Methods: For 30 days during the Spring 2017 semester, participants received prompts to participate in daily diaries about their previous day's sexual behavior on their smartphones. Participants then retrospectively reported their past 30 days of sexual behavior and provided feedback on the process of receiving daily diaries on their smartphones. Results: We found that college students overreported their sexual behavior on the retrospective survey compared to their daily diary reports (ps < 0.001; Cohen's ds = 1.51). Participants provided positive and constructive feedback. Conclusions: Using smartphones to administer daily diaries is a promising technique for obtaining reliable sexual behavior data from college students.