This study examined the relationship between infidelity and marital stability among married persons in Delta State, Nigeria. Five research questions and three null hypotheses guided the study. A correlational survey research design was adopted. The population comprised 8,889,293 married persons in Delta State (Federal Republic of Nigeria, 2015). A sample of 250 respondents was selected using multistage sampling involving purposive and simple random techniques. Data were collected using the Infidelity and Marital Stability Questionnaire (ISIMSQ), which included four subscales measured on a four-point scale: Sexual Infidelity (19 items), Social Infidelity (20 items), Emotional Infidelity (13 items), and Marital Stability (16 items). Face, content, and construct validity were established through expert review and factor analysis. Cumulative variances were 68.67% (Sexual), 55.43% (Emotional), with acceptable rotated factor loadings (≥ .40) across all scales. Reliability testing using Cronbach’s alpha yielded an overall coefficient of 0.81, with subscale values ranging from 0.73 to 0.85. Data were analyzed using mean, standard deviation, Pearson correlation, multiple regression, and path analysis at a 0.05 significance level. Findings showed that (i) the prevailing degree of marital stability among married persons in Delta State is high; (ii) the prevalence of infidelity among married persons is low; (iii) there is no significant relationship between sexual infidelity and marital stability; (iv) there is a significant relationship between emotional infidelity and marital stability; and (v) there is no significant relationship between social infidelity and marital stability. The study recommends programs that strengthen emotional intimacy, responsiveness, and mutual attentiveness among couples to sustain marital stability.