The incident occurred on Thursday afternoon.
The deceased was identified as Shah Md Shahriar Chowdhury, son of Shah Md Abdul Mannan Chowdhury of Kagapasha village of the upazila.
According to his cousin Parvez Chowdhury, the boy was playing at a field near their house Thursday afternoon when the snake bit him.
Initially he did not take it seriously and wandered for an hour before taking medicine from a local pharmacy, the cousin said.
As his condition worsened, he was rushed to Nabiganj Upazila Health Complex and later to Sylhet Osmani Medical College Hospital where physicians declared him dead, reports UNB.