We examined collective efficacy beliefs, including levels of within-group agreement and correlation with performance quality, of instrumental chamber ensembles (70 musicians, representing 18 ensembles). Participants were drawn from collegiate programs and intensive summer music festivals located in the northwestern and western regions of the United States. Individuals completed a five-item survey gauging confidence in their group's performance abilities; each ensemble's aggregated results represented its collective efficacy score. Ensembles provided a video-recorded performance excerpt that was rated by a panel of four string specialists. Analyses revealed moderately strong levels of collective efficacy belief and uniformly high within-group agreement. There was a significant, moderately strong correlation between collective efficacy belief and within-group agreement (r[subscript s] = 0.67, p < 0.01). We found no relationship between collective efficacy belief and performance quality across the total sample, but those factors correlated significantly for festival-based ensembles (r[subscript s] = 0.82, p < 0.05). Reliability estimates suggest that our collective efficacy survey may be suitable for use with string chamber ensembles. Correlational findings provide partial support for the theorized link between efficacy belief and performance quality in chamber music settings, suggesting the importance for music educators to ensure that positive efficacy beliefs become well founded through quality instruction.