This work aimed to evaluate the analgesic efficacy of intra-articular injection of magnesium sulphate (4%) compared with equivalent volume of bupivacaine (0.5%) after outpatient knee arthroscopic meniscectomy. Forty patients were randomly assigned to two groups. Group M (n = 20) received intra-articular magnesium sulphate 4%, group B (n = 20) received bupivacaine (0.5%). Analgesic effect was evaluated by analgesic duration, and by measuring pain intensity at 1, 2, 4, 6, 12, 24 h both at rest and on knee movement to 90°. The primary outcome variable was pain intensity on the VAS at 1, 2, 4, 6, 12, 24 h post arthroscopy at rest and on movement (flexion of knee to 90°), although the magnesium group had lower time weighted averages (TWAs) at rest and on movement, these TWAs were not statistically significant. The median duration of postoperative analgesia was significantly longer in the patients treated with magnesium sulphate (528 min) than in the bupivacaine group (317 min) (p < 0.0001), with less number of patients needing supplementary analgesia in magnesium group (8/20) than those of the bupivacaine group (16/20) (p < 0.022). Also analgesic consumption was significantly lower in the magnesium sulphate group (p < 0.002). We concluded that the use of magnesium sulphate is rational and effective in reducing pain, and is more physiological and shortens convalescence after outpatient arthroscopic meniscectomy, however our hypotheses that analgesic efficacy of intra-articular isotonic magnesium sulphate would be superior to intra-articular local anaesthetic cannot be supported with this study.