Perineural hydrodissection is a minimally invasive technique using an injection of fluid to dissect the perineural plane and tissue space. This report describes a case of palsy of the descending branch of the posterior interosseous nerve (PIN) which was recovered by targeted ultrasound-guided perineural hydrodissection. Ultrasonographic examination was performed, and multiple stenotic lesions interrupted by hyperechoic bands within the fascicles of the PIN were found. Using ultrasonography, perineural hydrodissection was performed four times every other week. Fifteen weeks after the first hydrodissection, there was no restriction in the patient’s thumb and fingers movement, and ultrasonography revealed that multiple stenotic lesions had improved. Today, surgical treatment is recommended for patients with complete nerve constriction. However, there is no standardized approach for patients with incomplete or without nerve constriction. Ultrasound-guided perineural hydrodissection is a noninvasive and easy method. This procedure could be a useful diagnostic and therapeutic modality for the management of the disease.