Background/Objectives: Posterior neck, trapezius, and interscapular pain, exacerbated by poor posture such as forward head and rounded shoulders, is common. In this study, we aimed to assess the clinical outcomes of isotonic saline injections at nerve entrapment points (NEPs) within the sternocleidomastoid (SCM) and scalenus medius (SM) muscles for alleviating spinal accessory nerve (SAN) and dorsal scapular nerve (DSN) compression in patients suffering from posterior neck, trapezius, and interscapular pain. Methods: In this retrospective study, 68 patients were included, with 34 receiving isotonic saline injections and 34 undergoing Extracorporeal Shock Wave Therapy (ESWT) as a control. The clinical outcomes were evaluated using the Visual Analog Scale (VAS) and Percent Pain Intensity Difference (PPID) before and after therapy. The effectiveness of isotonic saline injections targeting NEPs in the SCM and SM muscles in relieving pain associated with SAN and DSN entrapment was assessed. Results: Both treatments significantly reduced VAS and PPID scores, with injection therapy showing a larger treatment effect size (Cohen’s d: 3.521 for VAS and 3.521 for PPID) compared to ESWT (Cohen’s d: 1.379 for VAS and 1.710 for PPID). The mean clinically important difference observed for VAS was 4.2, exceeding the expected value of 2.6, indicating a substantial improvement in pain and patient quality of life. Conclusions: Isotonic saline injections at the NEPs of SAN and DSN within the SCM and SM muscles might potentially reduce posterior neck, trapezius, and interscapular pain associated with possible nerve entrapment, without causing complications. Further research is needed to validate these findings in larger, controlled trials.