The most photographed beaches on the south coast — main Mirissa, Unawatuna, Hikkaduwa — are genuinely beautiful. They are also genuinely busy, especially from December through March. If you want stretches of sand where you are not sharing space with fifty other tourists, you need to know where to look. These are the ones worth finding.
Secret Beach, Mirissa. About two kilometres from the main Mirissa beach, accessible via a narrow road that most tuk-tuk drivers know. It used to be genuinely secret. It is not anymore. But it is still significantly quieter than the main beach, and the setting is different: a protected blue lagoon, palm trees close to the water, and a small cove called Palm Cove around the rocks where the snorkelling is good. A couple of restaurants have set up there and prices are high, so bring your own water and snacks if you want to spend a few hours. Go early morning, before midday, and you will mostly have it to yourself.
Polhena Beach, near Matara. This is the best beach on the coast for snorkelling with wild sea turtles, and most tourists driving the coast road between Mirissa and Tangalle skip it entirely. Polhena sits just outside Matara town, about 30 minutes east of Mirissa by road. The beach is protected by a natural offshore coral reef that keeps the water calm and shallow, almost like a lagoon. Green turtles feed on the reef regularly and can be seen from the shore. No boat required. Snorkel gear can be rented from local vendors on the beach. It is busy with local families on weekends, but quiet on weekday mornings.
Madiha Beach, just past Polhena. One minute further east from Polhena, Madiha is a small village beach that barely appears on most tourist itineraries. The surf here is consistent and the beach is calm. There are a few local guesthouses and small restaurants. It has the feeling that Mirissa had ten years ago, which is either a recommendation or a warning depending on what you prefer.
Talalla Beach. Between Hiriketiya and Tangalle, Talalla is a long wide bay that is unhurried in a way the more famous beaches are not. It has a left-hand surf break that works well in the right swell. The village is small, the accommodation options are mostly small guesthouses rather than resorts, and the beach itself has almost no commercial development on it. It is one of the genuinely underdeveloped stretches of coastline left on the south coast.
Jungle Beach, Unawatuna. Not a hidden beach in name, but the journey to it filters out most of the day-trippers. From the Unawatuna road, there is a forest trail that takes about ten minutes on foot. At the end of it is a small cove with clear water, good for swimming, and far fewer people than the main Unawatuna beach ten minutes behind you. Go in the morning. Take water.
One honest note on the phrase "hidden beaches." Any beach described as hidden in a travel blog, including this one, has already been found. What these places offer is not secrecy. They offer a different pace, less infrastructure, and fewer people than the main spots. That is worth seeking out even if the beach is no longer a secret.