Yes, in order to protect your health, you do need a prescription to buy Viagra.

Online pharmacies should never provide prescribed medications without a prescription. Many online pharmacies provide on-staff physicians who review an online questionnaire that contains your comprehensive health/medical information. The doctor must also know what medications you currently consumer in order to write you a prescription.

If you order online, your prescription is typically filled by a pharmacy before it is shipped to you. Be sure that you are purchasing FDA-approved medications from a U.S. online pharmacy that partners with U.S.-licensed physicians and pharmacists. 

It is always recommended that you see your local doctor at least once a year in order to monitor your health.