ImportanceMusic listening is a universal human experience. People of all ages and cultures often use music to reduce stress and improve mood, particularly in times of crisis. However, ecologically valid research examining the real-time association of music listening with stress and mood during the COVID-19 pandemic is scarce.ObjectiveTo explore the associations between listening to music and the perceptions of stress and mood using ecological momentary assessment during the COVID-19 lockdown period.Design, Setting, and ParticipantsIn this cohort study conducted between April 1 and May 8, 2020, adults from the general population residing in Austria and Italy were prompted by an app on their smartphone to report data 5 times per day across 7 consecutive days. Participants provided data on their real-time and real-life experiences in their natural environment while strict lockdown measures were in place. Data analysis was performed from March 2021 to February 2022.ExposuresData on self-reported music listening were recorded by means of mobile-based assessments. Perceived chronic stress was assessed once at the end of the study.Main Outcomes and MeasuresPerceptions of momentary stress and mood were measured using visual analog scales (score range, 0-100, where 0 indicates not at all and 100 indicates very much) by means of mobile app–based assessments.ResultsThe final sample comprised 711 participants (497 women [69.9%]; median age, 27.0 years [IQR, 24.0-36.0 years]). Participants provided a total of 19 641 data points, including 4677 music listening reports. Music listening was prospectively associated with lower momentary stress levels (β, −0.92; 95% CI, −1.80 to −0.04; P = .04) and improvements in mood valence (β, 1.90; 95% CI, 1.17-2.63; P < .001), especially if the music was perceived as happy. Individuals with higher levels of chronic stress reported improved mood valence after music listening (β, 0.12; 95% CI, 0.02-0.22; P = .02).Conclusions and RelevanceThe present findings suggest that music listening may be a means to modulate stress and mood during psychologically demanding periods. Individuals experiencing heightened momentary and/or chronic stress because of the challenges brought about by COVID-19 pandemic–related restrictions might consider music as an easily accessible tool for the management of stress and mood in daily life.