<p>Financial transparency is a foundation of trust and stability, yet financial deception remains understudied. Using longitudinal, dyadic data from the CREATE project (<i>N</i> = 1,442), this study examined whether attachment insecurity (anxious and avoidant) intensified or explained the effects of financial deception on marital outcomes. Financial deception was assessed using a single-item measure. Although effect sizes were small, results were consistent across partners: greater financial deception by one partner predicted lower marital trust in the other partner one year later for both husbands and wives. Mediation analyses indicated that the associations were consistently explained by increases in the other partner’s avoidant attachment, which linked financial deception to lower trust and greater instability. Moderation analyses showed that husbands’ financial deception was more strongly associated with lower wives’ marital trust when wives reported higher anxious or avoidant attachment, and that wives’ financial deception was more strongly associated with husbands’ marital instability when husbands reported higher avoidant attachment. These results suggest that financial deception can quietly damage a marriage’s trust and stability, especially when attachment security is already fragile—in addition to the deception itself exacerbating attachment insecurity. Helping couples improve financial transparency and emotional security may reduce conflict and strengthen their relationship.</p>