Will Bradley Roland, a cameraman working with Indymedia New York, was shot in the chest and died before reaching hospital, the independent news group said on its website.
The journalist died after a bullet hit him in the torso in one of at least two shootouts in the city.
Oaxaca's state prosecutor's office said two others, including a protesting teacher, were also shot dead. The teacher, Emilio Alonso, was shot in a separate attack.
Their deaths add to a growing toll in the region, nine people, mostly protesters, have been killed in the conflict which began in Oaxaca state five months ago, when striking teachers and leftist activists occupied much of the state capital, a popular tourist destination.
Several people were wounded in the shootings on Friday.
A Reuters photographer said gunfire rattled for more than an hour near barricades at the edge of the city, famous for its colonial architecture, thriving arts scene and indigenous culture.
Earlier in the day, the activists blocked highways near the city and burned vehicles.
Flights to the Oaxaca were cancelled amid worries that protesters would occupy the city's airport.
This week, striking teachers voted to return to classes but many protesters say they will not back down until state Governor Ulises Ruiz is ousted.
Critics accuse the governor of corruption and repressive tactics against dissenters, whose roadblocks have driven tourism from the city and hurt business.
Mr Ruiz refuses to stand down and says he still controls Oaxaca, where no uniformed police have patrolled since being fought off by protesters during a failed attempt in June to break up a protest camp in the city's graceful central square.
President Vicente Fox has vowed to end the conflict before he leaves office on December 1, but negotiations to find a peaceful way out have so far failed.