Background: Interscalene brachial plexus block (ISB) provides excellent analgesia for arthroscopic shoulder surgeries but is associated with adverse effects including hemidiaphragmatic paresis. We aimed to compare the respiratory effects, forced vital capacity (FVC), and forced expiratory volume in 1 second (FEV1) between suprascapular nerve block (SSB) and ISB. Methods: Sixty patients were recruited and randomized into ISB, anterior SSB, and posterior SSB groups. FVC, FEV1, and diaphragmatic excursion were evaluated at baseline and 30 minutes after intervention. Blocks were performed under ultrasound guidance with 15 ml of 0.5% ropivacaine. Pain scores were assessed at 6, 12, and 24 hours postoperatively. Results: The ISB group showed a reduced FVC of 31.2% ± 17.5% (mean ± SD), while the anterior and posterior SSB groups had less reduction of 3.6% ± 18.6% and 6.8% ± 6.5%, respectively (P < 0.001). The ISB group showed more reduction in diaphragmatic excursion than the anterior and posterior SSB groups (median [IQR]): −85.7% (−95.3% to −63.3%) vs. −1.8% (−13.1% to 2.3%) and −1.2% (−8.8% to 16.8%), respectively (P < 0.001). The median pain scores (IQR) in the ISB and anterior SSB groups were lower than those in the posterior SSB group at 6 hours on movement: 0 (0–2), 1.8 (0–4.5) vs. 5 (2.5–8), respectively (P = 0.002). There was no significant difference in oxycodone consumption postoperatively. Conclusions Anterior SSB preserves lung function and has a comparable analgesic effect as ISB. Thus, it is recommended for arthroscopic shoulder surgeries, especially in patients who have reduced lung function.