Hypertension (HT) is a growing burden worldwide, leading to over 10 million deaths each year. In Brazzaville, the prevalence of HT was 32.5% in 2004. The mortality for stroke in 2008 and heart failure in 2013 were, respectively, 24% and 20.2%. May Measurement Month (MMM) is a global initiative initiated by the International Society of Hypertension aimed at raising awareness of HT and to act as a temporary solution to the lack of screening programmes worldwide. An opportunistic cross-sectional survey of volunteers aged ≥18 was carried out in May 2017. Blood pressure (BP) measurement, the definition of HT and statistical analysis followed the standard MMM protocol. The screening was carried out in Brazzaville, and the screening sites were distributed in different districts of the city in public places and health facilities. A total of 3842 individuals were screened during MMM17. After multiple imputations, 1576 (41.0%) had HT. About 956 (29.7%) individuals not receiving anti-hypertensive medication, were hypertensive. Four hundred and nine (66.0%) individuals receiving anti-hypertensive medication, had uncontrolled BP. Systolic and diastolic BPs after adjustment for age and sex differed significantly in association with use of anti-hypertensive medication (P < 0.0001), previous stroke (P = 0.001 for systolic), and waist circumference (P < 0.0001). MMM17 was the largest BP screening campaign undertaken in Congo. Almost one-third of screenees had untreated HT, and two-thirds of treated hypertensives were not well controlled. These results suggest that opportunistic screening can identify significant numbers with raised BP.