There hasn't been an official sequel to the 2006 film. However, a spin-off titled "Immortals" (2011) was released, which also depicted the Battle of Thermopylae but from a different perspective, focusing more on the mythological aspects.
For Tamil-speaking audiences, the film gained significant popularity on Tamilyogi due to its immersive dubbed audio. 300: Rise of an Empire (2014) - IMDb 300 spartans 2 tamilyogi
– The ground combat remains gloriously brutal, with slow‑motion “bullet‑time” strikes that are a signature Snyder move. The naval battles, however, are where the film shines: massive waves, exploding triremes, and an almost balletic coordination of ships that makes the sea feel like a battlefield in its own right. There hasn't been an official sequel to the 2006 film
To satisfy your curiosity about the actual sequel, here is a comprehensive breakdown. 300: Rise of an Empire (2014) - IMDb