Knee stiffness following anterior cruciate ligament (ACL) reconstruction remains a common complication, which can substantially impair knee function. The aim of this study was to assess the effectiveness of arthroscopic arthrolysis, in conjunction with manipulation under anaesthetic (MUA), in treating stiffness post-ACL reconstruction. We reviewed the records of 18 patients who underwent arthroscopic arthrolysis to treat established stiffness following primary isolated ACL reconstruction. Eight of these patients underwent concomitant MUA at time of arthrolysis. The median time between reconstruction and arthrolysis was nine months. Seven patients had arthrolysis performed within eight months of reconstruction, while 11 patients underwent arthrolysis greater than eight months postreconstruction. Following arthrolysis, the mean extension loss improved from 7° to 1°. In patients with mild extension stiffness (prearthrolysis extension deficits <10°), the mean improvement to extension was 3°. In patients with severe extension stiffness (prearthrolysis extension deficit ≥10°), the mean improvement to extension was 10°. Arthroscopic arthrolysis was significantly more effective in restoring extension loss if carried out within eight months of the primary reconstruction (P < 0.03). In the patients who underwent MUA at time of arthrolysis, the mean flexion loss improved from 16° to 4°. In conclusion, arthroscopic arthrolysis, in conjunction with MUA, is an effective treatment for knee stiffness post-ACL reconstruction but ideally should be carried out within eight months.