It the first breeding success there in a decade.
The male bird was the only chick to hatch from a clutch of five eggs and is one of only seven to have fledged in the whole of England this summer.
The other chicks fledged from two nests in Northumberland.
Says the RSPB's hen harrier project manager, Blanaid Denman: "The female parent arrived at Geltsdale back in May looking for a mate but she had to wait several weeks for a male to turn up.
"When one finally did appear, she was not impressed - he was young and had yet to gain his adult grey plumage.
"Normally, in a healthy population of hen harriers, an immature male like this wouldn't get a look in. But with so few birds in England, the female had little option but to accept his advances or leave breeding to another year.”
Once it had been confirmed there was a nest, RSPB staff and volunteers mounted a 24/7 watch and provided supplementary food under licence to ensure that the family of hen harriers had the best possible chance of survival and success.
Continues Blanaid: “The supplementary food proved vital as the inexperienced male was hopelessly inattentive of his dependent female, often vanishing for days before reappearing with a paltry food offering.
The extra food ensured that the female never had to go far from the nest to feed the chick or herself.”
The chick, named "Bonny", has been satellite-tagged and his movements will be monitored once he has left Geltsdale in a few weeks' time.
The RSPB says it will become possible to follow his movements online at at rspb.org.uk/henharrierlife