There is a need to constantly assess the awareness, treatment, and control of hypertension in Nigeria. This study determined the frequency of undiagnosed hypertension across the six geopolitical zones of Nigeria. We conducted an opportunistic screening of adults aged at least 18 years in the month of May 2021. Participants were recruited by trained volunteers using the May Measurement Month protocol. Blood pressure (BP) was measured using validated digital sphygmomanometers. We defined hypertension as systolic BP ≥ 140 and/or diastolic BP ≥ 90 mmHg and/or the use of BP-lowering medications. A total of 9361 participants (51.5% females) with a mean age of 40.7 ± 15.5 years were screened. Hypertension was present in 3192 (34.1%) of the participants. About half (1491, 46.7%) of the hypertensives were unaware of the diagnosis. Among the 3192 participants with hypertension, less than half (1540, 48.2%) were on antihypertensive medications, while only 36.4% of those on antihypertensive medications had their BP controlled (<140/90 mmHg). About one-third of Nigerians in this opportunistic screening had hypertension, with about half of them being unaware of their diagnosis while only about two out of every five on antihypertensive medications had controlled BP. Urgent health actions are needed in Nigeria to reduce the burden of hypertension and its complications.