The death is the first so far this year in Massachusetts from the mosquito-borne virus.
John Fontaine developed a fever Aug. 18 and was hospitalized two days later. He died Thursday afternoon, said Christine Powers-Perry, a spokeswoman for Children's Hospital in Boston.
The grade school student played youth football and was "like every other 9-year-old. Happy to be there. A joy to be around. Fun to watch grow," said Jim Muirhead, the program director for Mitchell Memorial Club Youth Football and Cheerleading program.
Middleborough was among the communities in southeastern Massachusetts that have undergone two rounds of aerial spraying to kill the mosquitoes that carry the virus. The first round of spraying Aug. 8 reduced the mosquito population by at least 60 percent, officials said.
The boy could have contracted the virus as early as Aug. 8, but probably got the virus the following week, according to state health officials.
Two other people - a 52-year-old woman from Lakeville and a 23-year-old man from Acushnet - have contracted the virus this year.
Four people have died in the state from the virus in the last four years. There were four human cases last year, all in Plymouth County.
The virus is transmitted to humans through mosquito bites. Symptoms range from mild flu-like illness to inflammation of the brain and coma. According to state health officials, about three of every 10 human cases is fatal.