May Measurement Month 2017 is a global initiative aimed at raising awareness of high blood pressure (BP) and to act as a temporary solution to the lack of screening programs worldwide, in which Spain participated actively. The primary objective was to raise awareness and increase control of BP in Spain. An opportunistic cross-sectional survey of volunteers aged ≥18 was set up in May 2017. Following the design of the International Society of Hypertension, data were collected from the 17 autonomous communities in which Spain is divided, mainly in community pharmacies, primary care centres and some hypertension (HT) units, and cardiovascular departments in hospitals. No additional training of volunteers was necessary. A total of 3849 individuals were screened. After multiple imputation, our data showed that 1923 (50.0%) had HT. In those not receiving antihypertensive medication, 17.5% were hypertensives, in individuals receiving antihypertensive medication, 33.9% had uncontrolled BP. May Measurement Month 2017 was the largest BP screening campaign undertaken in Spain. In total, 17.5% of people with HT did not receive medication. One-third of hypertensive participants receiving treatment did not have their BP controlled. These results confirm that an opportunistic screening can identify a significant number of subjects with and untreated and inadequately treated BP.