Yes, jellyfish are commonly found in the waters off Cascais and the rest of the Portuguese coast, particularly during the spring and summer months. The most frequent species is the Catostylus tagi, which is large but generally has a mild sting. However, the most concerning visitor is the Portuguese Man o' War (Physalia physalis), which is not a true jellyfish but a colonial organism. Its vibrant blue/purple float is easily spotted on the surface, but its nearly invisible tentacles can reach up to 30 meters (100 feet) in length and deliver a very painful sting that can cause skin lacerations and respiratory distress. Other species like the "Fried Egg" jellyfish and the "Moon Jelly" are also occasionally seen. Beachgoers in 2026 are advised to check the GelAvista app (Portugal's official jellyfish monitoring program) for real-time sightings. If stung, do not use fresh water, vinegar, or urine; instead, rinse the area with seawater and use a rigid object like a credit card to scrape off any remaining tentacles.