Jail officials told the Virginia-Pilot newspaper that Jamycheal Mitchell, 24, was being held there because he could not be placed with a mental hospital.
Police are investigating, but family members told the Guardian newspaper that Mr Mitchell had stopped eating.
He had a history of mental illness and had taken medication in the past.
Police in Portsmouth, Virginia, arrested Mr Mitchell on 22 April for petty larceny. A store clerk at the 7-11 store told the Guardian that Mr Mitchell had stolen a bottle of soda, a candy bar and a small cake.
He was held in Portsmouth city jail and transferred to Hampton Roads Regional Jail on 11 May.
Portsmouth police spokeswoman Misty Holley told the BBC that Mr Mitchell was found unresponsive in his cell about 05:45 am local time (09:45 GMT) on 19 August and he was pronounced dead shortly after.
Mr Mitchell's past charges include petty theft, larceny and trespassing, stemming back to 2010, according to court records.
The Guardian, which first reported the case, spoke to Mr Mitchell's family who said "his body failed" and he lost up to 65lbs (29kg) in jail.
A medical examiner will determine Mr Mitchell's cause of death, Ms Holley said. She could not confirm reports that Mr Mitchell was starving himself or not taking medication.
"Once we get results of the autopsy, we will know if there is any foul play," she said. "That determines what routes we go."
A spokeswoman for the Hampton Roads Regional Jail declined to answer questions from the BBC on Friday, but the jail would release more information on Mr Mitchell's case on Monday.
A Virginia man who was arrested for stealing $5 (£3) worth of snacks has died in jail after being held there for more than three months.