Context: Limited evidence exists to demonstrate the effect of extrinsic factors, such as footwear worn or the testing environment, on performance of the modified balance error scoring system (mBESS) in the middle school age (10–14 y) population. Therefore, the purpose of our study was to investigate the effect of footwear types and testing environments on performance of the mBESS by middle school athletes. Design: Cross-sectional. Methods: In total, 2667 middle school athletes (55.9% boys and 44.1% girls; age = 12.3 [0.94] y) were administered the mBESS while wearing their self-selected footwear (barefoot, cleats, or shoes) either indoors (basketball court) or outdoors (football field or track). The number of errors committed (range = 0–10) during the double-leg, single-leg, and tandem stances of the mBESS were summed to calculate a total score (range = 0–30). Kruskal–Wallis tests were used to assess for differences among the footwear groups for each mBESS stance and the total score. Mann–Whitney U tests with calculated nonparametric effect sizes (r) were used to assess for differences between the footwear groups and testing environments when appropriate. Results: There were significant differences for the number of committed errors among the footwear groups in the single-leg (P < .001) and tandem (P < .001) stances of the mBESS and mBESS total scores (P < .001). Significantly fewer errors (better) were committed while wearing shoes compared with other footwear in the single-leg and tandem stances of the mBESS (Ps ≤ .032, r = .07–.13). Participants assessed indoors committed significantly fewer errors than those assessed outdoors in each stance of the mBESS (Ps ≤ .022, r = .04–.14). Lower (better) mBESS total scores were observed for participants while wearing shoes (Ps ≤ .002, r = .10–.15) or assessed indoors (P = .001, r = .14). Conclusions: Although our data suggest that the type of footwear worn and the testing environment have a significant effect on mBESS scores of middle school athletes, the magnitudes of these differences are negligible.