Graphic video from the scene shows a woman laying face down amid spatters of blood while a passerby presses a cloth on to a large wound on her back.
A security officer was also hurt while trying to arrest the attacker, and all five victims have now been taken to hospital via ambulance.
A security source initially told Reuters the victims included three Spanish women, however Spain said its citizens were not involved.
A knifeman has injured four foreign tourists and four locals in an attack in the Jordanian city of Jerash.
There were no further details, but the al-Ghad newspaper said the tourists were Mexican and suffered serious wounds.
The Jordanian police spokesman told A Jazeera that the attacker is under investigation to determine the reason for the attack.
It is not the first time tourist sites have been targeted by attacks in Jordan. 'It's a dagger, it's a dagger, there is a knife.
The motive for the attack, which took place around noon (1000 GMT) at the Roman ruins of Jerash, a popular attraction 50 kilometres (30 miles) from the capital Amman, remains unknown, Sartawi added. Please, help him now!'
Such incidents are rare in Jordan, with the kingdom considered one of the Middle East's safest tourist destinations.
Seven police officers, two Jordanian civilians and a Canadian tourist were killed in the attack.
Jerash is famed for its Roman ruins.