Marla Ruzicka, 27, the founder of the Campaign for Innocent Victims in Conflict, was driving behind a private security convoy when the bomb exploded on Saturday, security officials said.
Two others died in the attack - Ruzicka's Iraqi colleague Faiz Ali Salim and a French national of Czech origin whose identity was not immediately released - besides the bomber.
Ruzicka, who grew up in California, had worked frequently in Iraq and Afghanistan trying to uncover details on the number of civilian casualties in the wars and secure compensation for the families of victims.
She also spent time cataloguing the impact of war on communities, often running great risks to do so.
She is one of at least half a dozen humanitarian workers who have been killed in Iraq. Four American Baptists were shot dead in a drive-by shooting near the northern city of Mosul in March last year.
And Margaret Hassan, an aid worker born in Ireland who ran the Care International group in Iraq, was killed by her kidnappers after a month-long hostage ordeal in November last year.
The road to Baghdad's international airport is one of the most dangerous in the country, with almost daily bomb blasts and ambushes.
Elsewhere, seven Iraqi Kurdish civilians working on a US military base were kidnapped late on Sunday in central Iraq after leaving work, a police chief told AFP.
Armed men seized the seven Kurds after stopping their bus as they travelled home from the base in the Mansuria region to Khanaqin, some 180km northeast of Baghdad.
Iraqis working for foreign forces
are often targeted
The bus driver, whom the armed men beat up, alerted police.
Civilians working for foreign forces in Iraq are often targeted by fighters.
US soldiers wounded
Aljazeera has learned that a number of US soldiers were wounded and a military personnel carrier was destroyed in a car bomb attack on the highway east of the city of Rutba on Sunday.
A US helicopter was seen evacuating the wounded.
In the town of al-Qaim in western Iraq, US forces seized a number of government buildings and emptied nearby houses of inhabitants in order to use them as military headquarters.