The four adults and four children, aged two, five, eight, and 10, were all in good health, Spanish police told AFP on Saturday.
"After we received an anonymous call, yesterday (Friday), agents from the Spanish police opened the back of a refrigerated truck in a lay-by on the A23 motorway and found eight Iraqis inside, all in perfect health," police said.
None of the migrants carried identification, but police said they were able to determine that all were Iraqi nationals.
The occupants included one family of two adults and three children, another family of a woman with a two-year-old daughter, and a man travelling alone.
It wasn't immediately clear how long they had spent inside the truck, but police said they were on their way to Britain when their truck was intercepted in the eastern province of Teruel.
"We do not know yet how these Iraqi families came to be in Spain," police said.
The driver, a 37-year-old Romanian national, was arrested on charges of human trafficking.
According to Spanish media reports, a family coming from Iraq was found in a refrigerated truck on the same motorway and in the same province last month.
On August 27th, the decomposing bodies of 71 people were found inside a truck at the side of an Austrian motorway in a discovery which sparked a horrified response across Europe as it struggles with its worst migration crisis since World War II.
Investigations revealed that the migrants - mostly from Syria, Iraq and Afghanistan - had been picked up at Hungary's border with Serbia and transported to Austria via Budapest.