The 19-year-old, who wore a white head cover, a long-sleeved green top and black leggings and sported luminous green running spikes, received a generous ovation from a capacity crowd at the Olympic stadium as she trailed in last of the eight runners.
"It's an incredible experience," Attar, who has dual United States citizenship and is a student at Pepperdine University in Los Angeles, told reporters.
"It is the hugest honour to be here to represent the women of Saudi Arabia.
"It is an historic moment. I hope it will make a difference. It is a huge step forward. It's a really incredible experience."
Attar, who clocked two minutes 44.95 seconds - over 43 seconds behind heat winner Janeth Jepkosgei Busienei of Kenya - was the second Saudi woman to compete at the Games following judoka Wojdan Ali Seraj Abdulrahim Shaherkani.
The International Olympic Committee (IOC) had extended a special invitation to Shaherkani and Attar after it pressed Saudi Arabia to end its ban on female participation.
Some conservative Saudis had criticised their countrywomen's participation in London after Saudi Arabia broke with its practice of sending male-only teams to the world's biggest sporting event.
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