Banks customers’ service delivery is a key focus for commercial banks management in achieving targeted performance. Studies indicate low customer service delivery attributable to lack of business ethics among banks in Lagos State. Extant literature reveals little evidence on the efficacy of bank ethics in enhancing customer service delivery in banks. Hence, this study examined the effect of ethical behaviour on customer service delivery of selected quoted deposit money banks in Lagos state, Nigeria. Cross-sectional survey research design was adopted in the study. The population was 38,003 staff and 17,780 corporate customers of eight selected deposit money banks’ in Lagos State, Nigeria. A sample size of 494 for banks staff and 494 customers were determined using Cochran’s formula. Stratified proportionate sampling technique was adopted in the study. Data were collected using a structured and validated questionnaire. Cronbach’s Alpha coefficients for ethical behaviour was 0.72 while for customer service delivery was 0.76. The response rate for bank staff and corporate customer were 88% and 89% respectively. Data collected were analyzed using descriptive and inferential (linear regression) statistics. The finding revealed that ethical behaviours had a significant effect on customer service delivery of selected quoted deposit money banks in Lagos state, Nigeria (β = 0.644, t = 13.086, p<0.05). It was recommended that management of deposit money banks in Nigerian banking systems should intensify its efforts to enhance ethical behavior and practices to improve bank customer service delivery.