The bird, called Thor, was last monitored on October 3 at Goodber Common, near Salter in Lancashire.
But his tag suddenly stopped transmitting earlier this month in an area close to a managed driven grouse moor.
Earlier, conservation officers had monitored his movements, watching him stay close to the Forest of Bowland where he had hatched in the summer.
James Bray, RSPB’s Bowland project officer, was involved in monitoring the nests in Bowland over the summer.
He said: “Whilst we know that hen harrier mortality rates are high for young birds, if Thor had died naturally we would expect to find either his body or his tag – or both.
"His tag was functioning well before he disappeared, which sadly suggests there has been some kind of interference with it.”
A RSPB spokesman added: "The incident was reported to the police and RSPB Investigations staff searched the area of her last known fix but found no sign of the bird or his tag."
RSPB officers discovered in the summer that hen harriers were nesting in Bowland for the first time since 2015.
Three nests were found, with the parent birds producing 13 chicks between.
Thor is the fourth bird to disappear in just two months, following the similarly unexplained disappearances of Hilma, Octavia and Heulwen in August 2018 – birds that were also tagged as chicks earlier this summer.
Alarmingly, the last known fix for Thor is directly between the sites where tagged hen harriers Hope and Sky were last heard from before they disappeared in 2014, which is the last time that there were chicks successfully raised in the Forest of Bowland.
If you have any information, please contact Lancashire Police on 101.
If you find a wild bird which you suspect has been illegally killed, contact RSPB investigations on 01767 680551 or fill in the online form: https://www.rspb.org.uk/our-work/our-positions-and-campaigns/positions/wildbirdslaw/reportform.aspx