The hosts found the ‘Lions of Mesopotamia’ as impossible to break down as they had South Africa in their opening group A game at the Mane Garrincha Stadium in Brasilia on 4 August.
Neymar showed his frustration after failing to inspire his side who, unlike the nation's women's team, are struggling to cope with the pressure of winning their first Olympic gold medal.
The hosts almost fell behind in the eighth minute when Mohanad Abdulraheem hit the post for Iraq but Brazil had their chances. Neymar almost scored direct from a corner kick and Renato Augusto also hit the woodwork with a long-range shot.
Brazil, who could only draw with South Africa in their opening match, had more of the ball and pushed forward in droves as the match wore on but they could not find a way past the Iraqi rearguard. Brazil had 20 shots at goal to Iraq's eight but they lacked composure to open the scoring.
The Brazilians will look to beat Denmark in their final game at the Fonte Nova Arena in Salvador on Wednesday and hope the result of the South Africa and Iraq match at the Corinthians Arena goes their way to ensure they reach the quarterfinals. Denmark are on top of group A with four points after beating South Africa 1-0 at the Mane Garrincha Stadium. Brazil and Iraq have two points and South Africa are bottom with one point.
Elsewhere Erick Gutierrez scored four goals as defending champions Mexico thrashed Fiji 5-1 at the Fonte Nova Arena in group C, where Germany drew 3-3 with the Republic of Korea.
Serge Gnabry, who plays for Arsenal in the English Premier League, struck twice for Germany, including a crucial equaliser two minutes into injury time, while Son Heungmin, who plays for Arsenal's north London rivals Tottenham, also netted as the Koreans stayed top of the group.
Portugal qualified for the quarterfinals after beating Honduras 2-1 at the Olympic Stadium in group D, where Argentina also won 2-1 against Algeria.
Nigeria booked their spot in the quarterfinals as they followed up their opening-game 5-4 win over Japan with a 1-0 victory over Sweden in group B at the Amazonia Arena. In the other group B game, Japan drew 2-2 with Colombia.