The US Secret Service said the letter was identified at a remote facility where White House post is screened.
It arrived at the facility on 16 April, the same day a letter intended for a US senator tested positive for the lethal toxin ricin.
That letter had been intended for Roger Wicker, a Republican senator representing Mississippi.
A spokesman for the Secret Service, which protects the US president and his family, said it was liaising with the US Capitol Police and Federal Bureau of Investigation (FBI) to trace the origins of the letter.
"This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery," Edwin Donovan said.
ABC are quoting sources saying the letter addressed to the president has tested positive for ricin but that the contents would be sent to an accredited laboratory for further analysis, as the preliminary results need further verification.
Ricin, extracted from castor beans, is 1,000 times more toxic than cyanide.
It can be fatal when inhaled, swallowed or injected, although it is possible to recover from exposure.
Correspondents say there is a heightened sense of alert in the capital after the deadly bomb attacks at the Boston marathon but there is no indication so far of any connection between the two incidents.