This study primarily aimed to report the outcome of the arthroscopic cancellous bone grafting technique for treating scaphoid nonunion. The secondary aim was to compare the bone union rate and time, and the ability to correct the angulation of the scaphoid, between arthroscopically treated patients (n = 27), and patients treated with open cancellous bone grafting (n = 27). Nine surgeons (two in the arthroscopic and seven in the open group) with Level III experience operated on the patients. The pain score, grip strength and Quick Disability of the Arm, Shoulder and Hand score improved significantly after the arthroscopic treatment (p = 0.0001). Arthroscopic cancellous bone grafting achieved union in 25 out of 27 patients. A retrospective comparison with open cancellous bone grafting showed four nonunions. Arthroscopically treated patients healed 5.4 weeks faster compared with the open group (p = 0.033). Patients treated with open grafting had a higher risk of failure (odds ratio = 2.17), although this was not found significant (p = 0.39). The open method corrected the angulation deformity better (dorsal cortical angle correction of 4°) than the arthroscopic method (dorsal cortical angle correction of 1°), but this was not statistically significant neither within the groups (p = 0.55, p = 0.87) nor postoperatively between the groups (p = 0.98). The height-to-length ratio was not different before and after the surgery for the arthroscopic group (p = 0.44) and the open group (p = 0.27), or postoperatively between the groups (p = 0.44). Level of evidence: III