三头同盟（Triumvirate，源于拉丁语「triumvirātus」），指三个有权力的人占主导地位的政治体制，三位都是其中一名「三执政」（英语：triumvir）。这项安排可以是正式的或非正式的，通常三人的权力表面平等，但在现实中这种情况很少见。这个词也可以用来描述三个不同的领导人都宣称自己是唯一的领导者的状态。

历史上著名的三头同盟通常是指：

A triumvirate (from Latin, triumvirātus, from trēs three + vir man) is a political regime dominated by three powerful individuals, each a triumvir (pl. triumvirs or triumviri). The arrangement can be formal or informal, and though the three are usually equal on paper, in reality this is rarely the case. The term can also be used to describe a state with three different military leaders who all claim to be the sole leader.