Cemal Ugur, a 36-year-old father of seven, was transporting what the US calls "supplies" to its forces in Iraq when he was killed outside Baghdad, Davut Ugur told reporters on Tuesday.
Cemal was last seen two months ago before Davut discovered his body in a Baghdad mortuary earlier this week.
He learned his brother had been killed after establishing contact with a group operating on the city's outskirts. Davut said he went to Iraq after the Turkish Foreign Ministry said it was unable to help his family.
"I met a representative of the resistance fighters who told me 'everyone bringing US soldiers equipment, is our enemy,' but they said they were not holding my brother," Davut Ugur said.
He then learned the group had shot Cemal after stopping him on a road outside Baghdad and wanted the cargo to be unloaded. They then set the truck alight. It was not clear when Cemal had been killed.
He found Cemal's body during a search of hospital mortuaries in the Iraqi capital and brought the corpse back to Turkey for a coroner's examination on Monday.
Scores of foreign workers in Iraq have been kidnapped by resistance fighters since April, including several Turkish truck drivers and construction workers. Some hostages have been released, but many of them have been killed in protest or due to their captors' demands not being met.