Cytomegalovirus (CMV) is an important pathogen in immunocompromised individuals. Coupled with Human Immunodeficiency Virus (HIV), it causes end organ diseases leading to increased morbidity and mortality in the population. The prevalence of Cytomegalovirus infection is above 93% in HIV infected children in Kenya. Despite, a high Cytomegalovirus seroprevalence found in children, few studies have documented CMV in adults. This study was done to determine the seroprevalence of CMV infection and its associated risk factors among HIV patients attending Thika level 5 Hospital in Kiambu County, Kenya. The study also evaluated the effect CMV infection on the immunity of HIV infected patients. A cross-sectional study involving 163 HIV positive participants from different age groups was carried out. A questionnaire was used to assess the socio-demographic and specific risk factors associated with cytomegalovirus. Blood was collected and analyzed for CD4 counts, CMV IgG and IgM. The seroprevalence of CMV was found to be 89% (CMV IgG) while the incidence was 10.4% (CMV IgM). The study found that CMV infection leads to more suppression of the immunity among the HIV infected patients. In addition, education, economic status, having other sexual transmitted infections, sharing drinks, immune status and blood transfusion were associated with CMV infection (p < 0.05). The study recommends adoption of CMV screening services and education on CMV risk factors as CMV infection preventive strategies.