Viana do Castelo, Portugal offers 9 beaches perfect for sunbathing, surfing, swimming, and more. Viana do Castelo is known for its beautiful beaches and rich cultural heritage. The best time to travel to Viana do Castelo, Portugal, is from June to September, when the average air temperature ranges from 20°C to 25°C (68°F to 77°F) and the average sea temperature is around 18°C to 20°C (64°F to 68°F). Sandee provides detailed maps, photos, and reviews from fellow beachgoers to help you Choose Your Beach™ and plan your perfect beach vacation!