The proportion of Malaysians of all ages who use private outpatient services has dropped over time, highlighting the overstretched condition of public outpatient facilities compared to their private counterparts. This paper aims to determine the prevalence of outpatient care, characteristics of outpatient care users by sector, and the factors affecting the utilisation of private outpatient services among the adult population of Malaysia using Andersen's behavioural model. Data from the National Health Morbidity Survey 2019 (NHMS 2019), a nationwide survey, were analysed. Logistic regression analysis was performed to explore the association of predisposing (locality, age, sex, ethnicity, education level, and marital status), enabling (working status, health care coverage, and household income), and need factors (perceived and evaluated needs) with the use of private outpatient services. Variables with a statistical significance ≤ 0.25 in the univariate regression analysis were included in the final multivariable logistic regression analysis. A total of 11,674 respondents, estimated to represent 22.4 million adults aged 18 years and above in Malaysia, were included for analysis. Overall, 8.3% of the adult population of Malaysia used outpatient care and 33.9% used the private sector. Those living in urban areas (OR = 1.80, 95% CI = 1.02, 3.18), non-Malays (OR = 1.74, 95% CI = 1.04, 2.93), those working (OR = 2.47, 95% CI = 1.48, 4.10), those with employer coverage (OR = 4.73, 95% CI = 2.79, 8.01), and those with health problems (OR = 2.26, 95% CI = 1.26, 4.05) were more likely to utilise private outpatient services. Those who self-rated their health status as fair (OR = 0.54, 95% CI = 0.33, 0.91) and who had diabetes, hypertension, or hypercholesterolemia (OR = 0.56, 95% CI = 0.31, 1.02) were less likely to utilise private outpatient services. The predisposing and enabling factors were associated with the use of private outpatient services, and the need factors were strong predictors of private outpatient care utilisation among adults. Understanding the factors associated with the utilisation of private outpatient services could aid in the development of effective initiatives designed to enhance outpatient care access among the population of Malaysia and balance the burden of outpatient care provision on the public and private sector.