Despite the government's commitment to the ease of doing business, there has been debate over why small and medium-sized enterprises are experiencing a downward trend in business survival. Statistics from the Economic Survey of 2021 show that small and medium-sized enterprises have grown significantly in Kenya during the past 10 years, accounting for around 96 percent of all business enterprises in the country. However, 90% of new businesses fail to survive past their third anniversary. The main objective of this study, therefore, was to investigate the effect of internal control systems on fraud detection and prevention among SMEs in Nairobi, Kenya; specifically to establish the effect of the control environment on fraud detection and prevention among SMEs in Nairobi, Kenya; to establish the effect of risk assessment mechanisms in fraud detection and prevention among SMEs in Nairobi Kenya; and, to establish the effect of monitoring in fraud detection and prevention among the SMEs in Nairobi Kenya. The study was anchored on Reliability and Fraud Diamond Theories. The study adopted an explanatory research design, also known as an experimental design. The group of interest was all the SMEs in the Nairobi Central Business District registered with Nairobi City County. According to the Nairobi City County License Register of 2021, there are 993 business enterprises in the Nairobi Central Business District with between 10 and 99 employees. Therefore, the target population of this study was 993 managers/owners of business enterprises in the Nairobi Central Business District. Yamane's (1967) formula was utilized to determine the sample size for the study in this circumstance. 400owners/managers from the target population served as the respondents for the study. Primary data was used in the study, and semistructured questionnaires were used to gather it. To evaluate the accuracy and dependability of the research instrument, a pilot test was carried out for the study. To assess this data, various data analysis techniques were used. Additionally, descriptive statistics, as well as inferential statistics, were used to examine the quantitativedata. SPSS version 28 was used to complete the data analysis task. The inferential statistics used were factor analysis and ordinal (ordered) logistic regression. The results were that there was a positive correlation between risk assessment and monitoring and fraud detection and prevention among SMEs in Nairobi Kenya. However, there was a weak correlation between the control environment and fraud detection and prevention among SMEs in Nairobi, Kenya. This study contributes to theory by building on the theoretical framework such as the Fraud Diamond theory and improving on the understanding of internal control systems, and the possible effect ICS could have on fraud detection and prevention. Empirically, the study guides management practices by giving insights on how to close the gaps in accounting requirements and other laws that SMEs employ to manipulate profits and deceive investors. The findings of this study were limited to the ICS components of the COSO framework. The study was also limited to data collected using an explanatory research design yet fraud detection and prevention may be affected by technological advancements and unprecedented occurrences which may affect how business is carried out in organizations.