The sub-adult hen harrier, which was satellite-tagged, and given the name John, stopped transmitting on Threshfield Moor on October 1.
The bird fledged in Northumberland in 2016 and was tagged in July of that year.
He wintered in in the same approximate area of Yorkshire in 2016/17, returned to Scotland and the Borders in spring/summer 2017, then back to Yorkshire in September 2017.
Natural England reported John’s disappearance to North Yorkshire Police. A search of the area has been conducted but nothing was found.
Hen harriers are considered the most persecuted bird of prey in the UK because of their unpopularity on grouse moors.
David Butterworth, Chief Executive of the Yorkshire Dales National Park Authority, said: “It is highly likely that a crime has taken place on Threshfield Moor.
"The spotlight is once again on the Yorkshire Dales as a black hole for raptors. This does no one any good.
"With colleagues in the Ranger service, I am doing all I can to support North Yorkshire Police. Any leads which the police might have had in the six weeks since this hen harrier disappeared have come to nothing, but we should not give up - someone must know something.
"I urge that person to contact the police.”
The RSPB’s Birdcrime report, released earlier this month, revealed at least 81 cases of persecution against so-called raptors in the UK last year.
But for the first time in three decades, there were no prosecutions.
The RSPB said North Yorkshire saw probably twice as much persecution of birds of prey as any other county over the last five years.
Last year's tally included four shot buzzards, four shot red kites, two poisoned red kites, a shot peregrine, a buzzard nest destruction and seven incidents relating to the illegal use of spring traps.
The loss of another juvenile hen harrier brings the total to six within fourteen months across northern England - and is a serious blow to the small English hen harrier population.
Sergeant Stuart Grainger, of North Yorkshire Police’s Rural Taskforce, said : “It is a disgrace that these beautiful birds appear to be relentlessly destroyed.
"The fate of this particular hen harrier remains unsolved at this time, but we are appealing for any information to assist the investigation.”
Rob Cooke, a Director at Natural England, said: “The sudden disappearance of the hen harrier, John, is a matter of grave concern. We urge anyone with information to get in touch with North Yorkshire Police.”