开放式关系（英语：open relationship）是人际关系的一种，处在这种关系中的双方有保持伴侣关系的意愿，但又不受传统的单配偶制的限制。这意味着双方同意保持恋爱关系或伴侣关系，同时也接受或者容许第三者的介入。开放式关系具体的状况相差很大，因为关系中包含的内容完全是由双方的意愿决定。有些处于开放式关系的一方只容许另一方与第三者发展关系，有些则能允许另一方与第三者约会、接吻甚至发生性行为。结合最普遍的定义来看，开放式关系的首要特征就是处在这种关系中的双方还和第三者保持短期（如与他人约会）或长期（如开放式婚姻）的恋爱关系。开放式关系的特性，决定了它的灵活性，双方的关系可能处在不断的变化中，与第三者的关系也有协调的余地，所以开放式关系也是可以发展的。

An open relationship is an interpersonal relationship in which the parties want to be together but agree to a form of a non-monogamous relationship. This means that they agree that a romantic or intimate relationship with another person is accepted, permitted, or tolerated. Generally, an open relationship is when the parties involved have two or more romantic or sexual relationships occurring at the same time either as a short-term relationship, such as dating, or a long-term relationship, such as marriage. The concept of an open relationship has been recognized since the 1970s.