There have been several contenders for the longest sea beach in the world.
1. The longest beach was said to be the one stretching from Adale in Somalia to the Horn of Africa, with a length of over 700 km. But this does not seem to be a long continuous sandy beach. It is a long coastline with rocky cliffs, et, in between.
2. The area containing Terence Bay in Namibia was often thought to be the second longest beach, stretching to 480 km. It is also known as the Skeleton Coast. But there seems to be no verified continuous beach in Namibia that extends to such a long stretch uninterruptedly.
3. The third position is often awarded to the Eucla beach in Australia. It is believed to be 280 km long. But the officially verified uninterrupted single stretch of that beach seems to be only 41.5 km.
So Praia do Cassino beach of Brazil, stretching 254 km or 158 miles from the Rio Grande to the border of Uruguay, seems to be the winner in the competition. It has an uninterrupted sandy beach extending to 254 km. It has also been considered the world's longest sandy beach by the Guinness Book of World Records after examining all other claims.