The boy is believed to have been taking part in a one-to-one football training session on the field at the end of School Road, South Shore, when he was struck at around 5.05pm today, May 11.
Police ambulance and air ambulance services were called to the scene, and he was taken to hospital with serious injuries.
Officers have now confirmed he has died.
A police spokesman said: "A nine-year-old boy has sadly died following an incident on a football field shortly after 5.05pm today.
“Although enquiries are still ongoing, we believe the boy was struck by lightning.
“Our thoughts are with the boy's family at this sad time."
Spirit of Youth FC secretary Daniel Pack said: "Unfortunately, we don't know much at the moment.
“We extend our sincerest condolences to the family involved.
“We are all very upset and saddened by this whole thing. The whole club is shocked."