The military said another soldier was wounded by the roadside bomb. The Diyala attack that took place on Monday was announced by the military on Tuesday.
The military had earlier announced that five US soldiers were killed and three wounded after a suicide attack earlier in the day in Baghdad.
The Baghdad attack, in Mansur neighbourhood, was the deadliest on American troops in two months.
Lieutenant Michael Street, a military spokesman, said on Monday: "Five US soldiers were killed today when their dismounted patrol was struck by a suicide bomber. Three US soldiers and an Iraqi interpreter were wounded as well."
The deaths took to 3,982 the number of US troops killed in Iraq since the US-led invasion in March 2003.
The last major attack against US troops took place on January 9 when six US soldiers were killed after they entered a booby-trapped house in Diyala.
Roadside bomb
An Iraqi security official said two civilians were also killed, and 15 others wounded in the suicide attack in Baghdad.
Witnesses said the bomber struck after a convoy of five or six military vehicles stopped at a shop.
US troops later cordoned off the neighbourhood. Some witnesses said the bomber was dressed in a suit and tie.
Troop reduction
Colonel Allen Batschelet, a US military spokesperson, said in a statement that the attack would not deter the military from working with Iraqi forces "to protect the Iraqi people against terrorists, extremists and criminals".
Last week, the US military announced a reduction in troops numbers in Iraq after saying that violence in the country has dropped.
About 2,000 soldiers from the 82nd Airborne Division, deployed in last year's mission, will not be replaced once they leave the country in a few weeks' time.
The US military currently has 158,000 troops in Iraq.