The 21-year-old died from a stab wound to the chest as he was taking pictures in Alexandria, amid warnings of a “civil war”.
Violent clashes across the Egyptian cities of Alexandria and Port Said have left three people dead and more than 70 others injured.
Two people were killed in Alexandria. One of them was an American citizen, the US State Department confirmed.
He has been identified as Andrew Pochter, 21, from Maryland, who was a student at Kenyon College in Ohio.
He had been working as an intern at Amideast, an American non-profit organization, a statement from the college said.
Mr Pochter died from a stab wound to the chest after violence erupted between supporters and opponents of President Mohamed Morsi.
General Amin Ezzeddin, a senior Alexandria security official, said the American was using a mobile phone camera near an office of Mr Morsi’s Muslim Brotherhood in the city’s Sidi Gaber neighborhood when it was being attacked by protesters.
He was rushed to a military hospital, where he died.
A second victim was shot dead during clashes in the city, while a third person died as protests also turned violent in Port Said.
The deaths come as leading clerics warned of “civil war” in Egypt after violence in the last week has left several dead and hundreds wounded.
They backed Mr Morsi’s offer to talk to opposition groups ahead of mass protests scheduled for Sunday.
State news agency MENA said 70 people had been injured.
TV footage showed protesters running from the scene as gunshots were heard.
The offices of the Freedom and Justice Party (FJP), the political arm of Mr Morsi’s party, were also set on fire during the confrontations.
A Brotherhood member was also killed overnight in an attack on a party office at Zagazig, in the heavily populated Nile Delta, where much of the recent violence has been concentrated.
Mr Morsi’s movement said five supporters in all had died this week – three in Mansura and two in Zagazig.
The unrest is seen by many as a prelude to mass anti-Morsi protests planned for Sunday – marking his first year in office.
The June 30 protest was called by Tamarod, a grassroots movement which says it has more than 15 million signatures for a petition demanding Mr Morsi’s resignation and a snap election.
It alleges that Mr Morsi reneged on his promise to be a president for all Egyptians and has failed to deliver on the uprising’s aspirations for freedom and social justice.
The president himself warned in a televised speech on Wednesday that the growing polarization threatens to “paralyze” Egypt.
The army, which oversaw the transition from former president Mubarak’s autocratic rule but has been on the sidelines since Mr Morsi’s election, warned it would intervene if violence erupts.
It has brought in reinforcements to key cities, security officials said.
In an updated travel warning, the State Department cautioned US citizens “to defer non-essential travel to Egypt at this time due to the continuing possibility of political and social unrest”.