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Fascial plane blocks are rapidly emerging to provide safe, feasible alternatives to epidural analgesia for thoracic and abdominal pain. We define a new option for chest wall and upper abdominal analgesia, termed the rhomboid intercostal and subserratus plane (RISS) block. The RISS tissue plane extends deep to the erector spinae muscle medially and deep to the serratus anterior muscle laterally. We describe a 2-part proof-of-concept study to validate the RISS block, including a cadaveric study to evaluate injectate spread and a retrospective case series to assess dermatomal coverage and analgesic efficacy.For the cadaveric portion of the study, bilateral ultrasound-guided RISS blocks were performed on 6 fresh cadavers with 30 mL of 0.5% methylcellulose with india ink. For the retrospective case series, we present 15 patients who underwent RISS block or RISS catheter insertion for heterogeneous indications including abdominal surgery, rib fractures, chest tube–associated pain, or postoperative incisional chest wall pain.In the cadaveric specimens, we identified staining of the lateral branches of the intercostal nerves from T3 to T9 reaching the posterior primary rami deep to the erector spinae muscle medially. In the clinical case series, dermatomal coverage was observed in the anterior hemithorax with visual analog pain scores less than 5 in patients who underwent both single-shot and continuous catheter infusions.Our preliminary cadaveric and clinical data suggest that RISS block anesthetizes the lateral cutaneous branches of the thoracic intercostal nerves and can be used in multiple clinical settings for chest wall and upper abdominal analgesia.