Cut 1 kg of watermelon into small cubes, removing the seeds and rind. Set aside in a large mixing bowl.
In a small bowl, mix 1 teaspoon chaat masala, 1/2 teaspoon black salt, 1/2 teaspoon red chili powder, and 1/2 teaspoon roasted cumin powder.
Pour the spice mixture over the watermelon cubes and mix gently to coat the watermelon evenly.
Add 2 tablespoons of finely chopped mint leaves and 2 tablespoons of finely chopped coriander leaves to the watermelon. Mix well.
Drizzle 3 tablespoons of tamarind chutney and 2 tablespoons of green chutney over the watermelon mixture. Gently toss to combine.
Transfer the watermelon chaat to a serving plate and garnish with a few more mint and coriander leaves. Serve immediately.