Significant other (sig ot, sigot, sigoth, SigO, or SO) is a gender-blind, politically correct term to refer to a person's partner in an intimate relationship without disclosing or presuming anything about his or her marital status or sexual orientation. It is also vague enough to avoid offence from using a term that an individual might consider inappropriate (e.g. lover when he or she considers him a boyfriend, or her a girlfriend when he or she considers her a life partner). Nonetheless, some are offended by the implication that persons with whom one is not having a "primary" sexual relationship are "insignificant" or would as a matter of course get lesser consideration and emotional disrespect compared to the "significant" one.

In the United States the term is commonly used in invitations, e.g. to weddings and office parties. In this context, a person and their significant other might not be cohabiting and might not be engaging in sexual relations. For example, a person's fiancé(e) would be considered a significant other, without any assumptions regarding living arrangements or sexual activity before their marriage. Conversely, it is assumed that some sort of steady connection exists between the invitee and their significant other, i.e. the event for which the invitation is extended won't be their first date.

The first known occurrence of the term was in 1953 by U.S. psychiatrist, Harry Stack Sullivan, a former editor of the journal Psychiatry, in his posthumously published work, The Interpersonal Theory of Psychiatry.

Significant Other is the second studio album by Limp Bizkit, released on June 22, 1999.