Clonidine, an alpha-2 adrenergic agonist, has been proposed as an antimanic agent that acts by reducing noradrenergic transmission. We conducted a systematic review to examine the efficacy and safety of clonidine for acute mania/hypomania. A comprehensive literature search was performed to identify randomized controlled trials (RCT) and non-randomized studies investigating the efficacy and safety of monotherapy/adjuvant treatment with clonidine for acute mania/hypomania in patients with bipolar disorder (BD). Nine studies (n = 222) met our inclusion criteria, including five RCTs (n = 159) and four non-randomized studies (n = 63). Non-randomized studies showed clonidine to help reduce symptoms of mania. However, data from placebo controlled RCTs were inconsistent. One RCT showed adjuvant clonidine as superior to placebo, whereas another RCT reported that clonidine was not better than placebo. In individual RCTs, lithium and valproate offered better antimanic effects compared to clonidine. Studies reported hypotension, depression, and somnolence as common adverse effects. Significant differences in study design and sample size contributed to high heterogeneity. This systematic review suggests low-grade evidence for clonidine as an adjuvant treatment for acute mania with mood stabilizers and inconclusive efficacy as monotherapy, warranting further well-designed RCTs.