The father of the victim, 43-year-old Jared Tucker, told US media his son's wife of only one year had identified his body at the consulate.
Father-of-three Mr Tucker and spouse Heidi Nunes had been enjoying drinks on the Las Ramblas promenade when a van drove into pedestrians.
Thirteen people died, and more than 100 were injured.
Earlier in the day, Secretary of State Rex Tillerson offered his condolences to the victim's loved ones, and said officials were "still confirming the death or injuries" of any other Americans.
Ms Nunes told NBC that Mr Tucker had excused himself to go to the bathroom minutes before the attack.
"Next thing I know there's screaming," Ms Nunes said.
"I got pushed inside the souvenir kiosk and stayed there hiding while everybody kept running by screaming."
The couple from Walnut Creek, California, were celebrating their first wedding anniversary and honeymoon by travelling through Europe, and had arrived in Spain after visiting Paris and Venice.
Jared's father, Dan Tucker, told KGO-TV that his family had not heard until Friday morning that Ms Nunes had been called to the US consulate, and "was shown pictures of his body".
"She confirmed it was him and now she's on the way over to the morgue to make a real confirmation," Mr Tucker said, adding: "He didn't make it."
The victim's father added that he and his son had worked together in the construction industry since he was 16 years old.
Jared Tucker leaves behind three teenage daughters.
The attack was one of three incidents in Spain, which officials have said were part of a "jihadist" plot.
People from 34 different countries were injured or killed, emergency services said.

The 30-year-old, riding for Team Dimension Data, won stage one to Al Khor Corniche in a sprint finish from Sacha Modolo (Lampre) and Andrea Guardini (Astana).
Cavendish, who won the race in 2013, averaged close to 52km/h on this stage.
He plans to decide during the race whether he will compete at next month's track World Championships in London.
A winner of 26 Tour de France stages, Cavendish faces a packed schedule in 2016 with August's Olympic Games in Rio also a target.
The five-stage race finishes on Friday.
An American honeymooner who died in Thursday's Barcelona terror attacks has been identified by his family.
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Britain's Mark Cavendish claimed his first win of 2016 on a fast opening stage of the Tour of Qatar.