Background: Several studies have investigated the effects of intrathecal magnesium sulfate as an adjuvant for bupivacaine; however, their conclusions are inconsistent. Therefore, it is necessary to conduct a meta-analysis on this topic.Methods We searched Pubmed, EMBASE (OvidSP) and Cochrane Central Register of Controlled Trials (CENTRAL) for randomized controlled trials (RCTs) comparing the effect of intrathecal bupivacaine combined with magnesium sulfate versus bupivacaine alone in adults using spinal anesthesia.Results Eighteen studies that met our inclusion criteria were included in our analysis. We found that the addition of intrathecal magnesium sulfate to bupivacaine provided a longer duration of analgesia (SMD 0.99; 95% CI [0.45, 1.52], P = 0.0003, I2 = 93%), prolonged the duration of sensory block (MD=106.69; 95% CI, 60.93-152.45; P<0.00001), delayed the onset of sensory block (SMD 1.20; 95% CI [0.65, 1.75], P =<0.0001, I2 = 91%) and motor block (SMD 1.46; 95% CI [0.23, 2.69], P =0.02, I2 = 96%), decreased the requirement for rescue analgesia (SMD -0.81; 95% CI [-1.06, -0.56], P < 0.00001, I2 = 11%). For duration of motor block, and incidence of postoperative adverse events (such as nausea and vomiting, hypotension, bradycardia, pruritus, shivering and neurological deficit), no statistically differences were observed between the 2 groups.Conclusions Our meta-analysis demonstrated that intrathecal magnesium sulfate combined with bupivacaine prolongs the dusration of analgesia, without an impact on the adverse events. However, the quality of evidence was very low when using GRADE to assess it. Given adverse effects before use, more high-quality trials with large samples are required before magnesium sulfate is routinely used as a intrathecal adjunct.