The relationship between narcissism, callousness, and extradyadic involvement (EDI), has been understudied. It was anticipated that callousness and narcissism would be associated with increased likelihood of engaging in an EDI and less distress about having engaged in the EDI. Participants (268 college students; 75% female; mean age of 20 years old) completed self-report sections of the Self Report Psychopathy Scale III (Williams, Paulhus, & Hare, 2007), Short Dark Triad (Jones & Paulhus, 2014), and the Extradyadic Experiences Questionnaire (Allen & Baucom, 2004). Young adults who engaged in an EDI reported more narcissism and callousness. As expected, more callous participants tended to report more gladness and less distress about having engaged in an EDI. Unexpectedly, narcissism was associated with less gladness and more guilt about having engaged in the EDI. Future studies should more stringently test the investment model of relationships by including measures of the mediating variables of commitment and perceived availability of romantic partners.