Infidelity is a relational process common in all types of romantic relationships and has been established as one of the main causes of relationship breakdown. However, little is known about this type of transgression in adolescent romantic relationships, although it manifests as a fairly frequent behavior involving different motivations. Even less is known about the emotional impact of infidelity on the offending person and its association with hostile behavior and psychological well-being. Through an experimental study (N = 301 Spanish adolescents (190 female and 111 male; Mage = 15.59, SD = 0.69; range from 15 to 17), we sought to analyze the effect of manipulating two types of motivations for infidelity (sexual vs. emotional dissatisfaction) on negative affect, hostility, and psychological well-being. The main results revealed that committing infidelity motivated by hypothetical sexual (vs. emotional) dissatisfaction was indirectly related to lower psychological well-being through its effects on increased negative affect and hostility. Last but not least, we discuss these findings, highlighting the possible implications of infidelity for the psychosocial and psychosexual development of adolescents.