This study examined whether two personality traits, attachment anxiety, and the Dark Triad, longitudinally promote stalking behaviors after romantic breakups through psychological maltreatment of partners during relationships and maladjusted reactions to breakups. We conducted a prospective longitudinal study across four waves of assessment for 1 year of individuals in romantic relationships in Japan. They completed measures of attachment anxiety, the Dark Triad, need for control, and psychological intimate partner violence (IPV) perpetration at Wave 1, and measures of maladjusted reactions to breakups (i.e., anger and rumination) and stalking behaviors at a wave after breakups. The final sample consisted of 356 participants who were involved in romantic relationships at Wave 1 and left by their partners during the survey period. Correlational analysis showed that the Dark Triad at Wave 1 was significantly positively associated with future stalking behaviors, but attachment anxiety was not. Structural equation modeling showed that attachment anxiety and the Dark Triad increased future stalking behaviors after breakups through psychological IPV perpetration during relationships. Additionally, attachment anxiety increased future stalking behaviors through higher levels of postbreakup anger and rumination. Conversely, we did not find any indirect effects of the Dark Triad on stalking behaviors through these reactions toward breakups. Overall, these results suggest that it is necessary to consider personality traits that could damage intimate relationships and to focus on the quality of romantic relationships to prevent stalking after romantic breakups. These results also indicate that emotion regulation may be useful in preventing stalking caused by attachment anxiety.