Peter Stoeker said there were marks on Jalal Sharafi's feet, legs, back and nose but he was unable to say if they were the result of torture.
Iranian media quoted Mr Sharafi saying the CIA tortured him "day and night".
Mr Sharafi was abducted in Iraq in February and released last week. The US denies any involvement in the case.
Mr Sharafi, second secretary at the Iranian embassy in Baghdad, says he was kidnapped by Iraqi agents operating under the supervision of the CIA.
Iranian state media has quoted Mr Sharafi saying the CIA subjected him to torture as they questioned him about Iranian assistance to groups inside Iraq.
'Evidence of torture'
Iranian television has shown pictures of Mr Sharafi receiving treatment in hospital and quotes a doctor's report saying there are signs someone drilled holes in his feet as well as broke his nose, injured his ear and wounded his neck and back.
The ICRC's Mr Stoeker said he had been happy to meet Mr Sharafi in hospital because his organisation had been unable to find him in Iraq.
He confirmed he saw wounds on Mr Sharafi's feet, legs, back and nose but, not being a doctor, he was unable to say if they were the result of torture and if so, who inflicted them.
"The United States had nothing to do with Mr Sharafi's detention and we welcome his return to Iran," said Gordon Johndroe, a White House spokesman, last week.
He dismissed the claims as "just the latest theatrics of a government trying to deflect attention away from its own unacceptable actions".
An unnamed US intelligence official also denied any claims of abuse, saying: "The CIA does not conduct or condone torture."
Mr Sharafi was freed in Iraq on 3 April, the day before the 15 British sailors were set free in Iran, but no link has been confirmed between the two cases.