An intimate relationship is an interpersonal relationship that involves emotional or physical closeness between people and can include feelings of romantic or platonic love and sexual intimacy. Intimate relationships are interdependent, and the members of the relationship mutually influence each other. The quality and nature of the relationship depends on the interactions between individuals, and is derived from the unique context and history that builds between people over time. Social and legal institutions such as marriage acknowledge and uphold intimate relationships between people. However, intimate relationships are not necessarily monogamous or sexual, and there is wide social and cultural variability in the norms and practices of intimacy between people.

The course of an intimate relationship includes a formation period prompted by interpersonal attraction and a growing sense of closeness and familiarity. Intimate relationships evolve over time as they are maintained, and members of the relationship may become more invested in and committed to the relationship. Healthy intimate relationships are beneficial for psychological and physical well-being and contribute to overall happiness in life. However, challenges including relationship conflict, external stressors, insecurity, and jealousy can disrupt the relationship and lead to distress and relationship dissolution.

Intimacy is the feeling of being in close, personal association with another person. Emotional intimacy is built through self-disclosure and responsive communication between people, and is critical for healthy psychological development and mental health. Emotional intimacy produces feelings of reciprocal trust, validation, vulnerability, and closeness between individuals.

Physical intimacy—including holding hands, hugging, kissing, and sex—promotes connection between people and is often a key component of romantic intimate relationships. Physical touch is correlated with relationship satisfaction and feelings of love. While many intimate relationships include a physical or sexual component, the potential to be sexual is not a requirement for the relationship to be intimate. For example, a queerplatonic relationship is a non-romantic intimate relationship that involves commitment and closeness beyond that of a friendship.

Among scholars, the definition of an intimate relationship is diverse and evolving. Some reserve the term for romantic relationships, whereas other scholars include friendship and familial relationships. In general, an intimate relationship is an interpersonal relationship in which physically or emotionally intimate experiences occur repeatedly over time.

Interpersonal attraction is the foundation of first impressions between potential intimate partners. Relationship scientists suggest that the romantic spark, or "chemistry", that occurs between people is a combination of physical attraction, personal qualities, and a build-up of positive interactions between people. Researchers find physical attractiveness to be the largest predictor of initial attraction. From an evolutionary perspective, this may be because people search for a partner (or potential mate) who displays indicators of good physical health. Yet, there is also evidence that couples in committed intimate relationships tend to match each other in physical attractiveness, and are rated as similarly physically attractive by both the members of the couple and by outside observers. An individual's perception of their own attractiveness may therefore influence who they see as a realistic partner.

Beyond physical appearance, people report desirable qualities they look for in a partner such as trustworthiness, warmth, and loyalty. However, these romantic ideals are not necessarily good predictors of actual attraction or relationship success. Research has found little evidence for the success of matching potential partners based on personality traits, suggesting that romantic chemistry involves more than compatibility of traits. Rather, repeated positive interactions between people and reciprocity of romantic interest seem to be key components in attraction and relationship formation. Reciprocal liking is most meaningful when it is displayed by someone who is selective about who they show liking to.

When potential intimate partners are getting to know each other, they employ a variety of strategies to increase closeness and gain information about whether the other person is a desirable partner. Self-disclosure, the process of revealing information about oneself, is a crucial aspect of building intimacy between people. Feelings of intimacy increase when a conversation partner is perceived as responsive and reciprocates self-disclosure, and people tend to like others who disclose emotional information to them. Other strategies used in the relationship formation stage include humor, initiating physical touch, and signaling availability and interest through eye contact, flirtatious body language, or playful interactions. Engaging in dating, courtship, or hookup culture as part of the relationship formation period allows individuals to explore different interpersonal connections before further investing in an intimate relationship.