Gwent Police made the grim find near the main road between Abergavenny and Blaenavon at midnight on Monday.
Formal identification is not expected until Tuesday afternoon at the earliest.
Senior crime officers, forensic archaeologists and search teams had been concentrating their search on the wooded area of Blaenavon Mountain called Fiddler's Elbow after new information was given to them on Monday afternoon.
Chief inspector John Oliver, of Gwent Police, said: "Specialist liaison officers continue to support Jenna's family at this difficult time.
"The past few weeks have been extremely traumatic for the entire family.
"Hopefully we will soon be in a position to draw to a close the events which have caused so much pain."
Woodland search
Police closed a half-mile stretch of the B4246 road between Abergavenny and Blaenavon while the fresh search was carried out.
Jenna's stepfather Mike Baldwin, 36, is in custody charged with the 15-year-old's murder.
She has been missing from the family home at Abersychan near Pontypool since 10 September.
Detectives had previously concentrated their search on woodland and a disused quarry in the Abersychan area using specially-trained dogs.
Teams from other police force areas have been drafted in to help.
A helicopter using thermal imaging equipment was used and sniffer dogs, specially trained to find human remains, were brought in.
Last week, police frogmen searched focused areas of water including the Afon Llwyd River and The Keeper's Pond at Blaenavon.
Detectives also appealed for information about a Bulldog brand shovel which they believed could be a vital clue in tracking down Jenna's body.