<b>Background:</b> To determine the effect of threshold inspiratory muscle training (IMT) on functional fitness and respiratory muscle strength (RMS) compared to incentive spirometry (IS) in children/adolescents with obesity. <b>Methods:</b> A total of 60 obese children/adolescents aged 8-15 years were randomized into the threshold IMT group (<i>n</i> = 20), the IS group (<i>n</i> = 20), or the control group (<i>n</i> = 20). The IMT group performed 30 inspiratory breaths with the intensity set at 40% of baseline maximal inspiratory pressure (MIP) twice daily for 8 weeks; the IS group performed 30 breaths with sustained maximum inspiration twice daily for 8 weeks; and, the control group was assigned no training device for 8 weeks. Six-min walk test (6-MWT), RMS, and spirometry were compared between baseline and 8 weeks. <b>Results:</b> Six-MWT distance (528.5 ± 36.2 vs. 561.5 ± 35.2 m, <i>p</i> = 0.002) and MIP (121.2 ± 26.8 vs. 135.3 ± 32.1%Predicted, <i>p</i> = 0.03) were significantly improved after 8 weeks of IMT training. There was no significant difference in any evaluated pulmonary function parameters between baseline and 8 weeks in the IS or control groups; however, 6-MWT distance demonstrated a trend toward significant improvement in the IS group (526.9 ± 59.1 vs.549.0 ± 50.6 m, <i>p</i> = 0.10). No significant difference among groups was found for any variable relative to change from baseline to post-training. <b>Conclusions:</b> Eight weeks of threshold IMT training significantly improved both inspiratory muscle strength (MIP) and functional fitness (6-MWT) in children/adolescents with obesity. Eight weeks of IS training yielded a trend toward significantly improved functional fitness.