The antibody neutralised all known Zika strains in lab experiments, and "markedly reduced" virus levels in infected pregnant mice and theirfoetuses.
"This is the first antiviral that has been shown to work in pregnancy to protect developing foetuses from Zika virus," said study co-author Michael Diamond of the Washington University School of Medicine.
"This is proof of principle that Zika virus during pregnancy is treatable and that we already have a human antibody that treats it, at least in mice."
Zika is spread mainly by mosquitoes, but in rare cases via sex.
In most people it is benign with mild or no symptoms. But in an outbreak that started in 2015, it has been linked to microcephaly in foetuses -- a condition which restricts head growth and causes brain damage.
More 2,300 babies, mainly in Brazil, have been born with microcephaly or other Zika-related central nervous system malformations since last year, according to the World Health Organization (WHO).
There is no cure or vaccine.
Diamond and a team trawled through antibody samples from people who had recovered from a Zika infection.
One called ZIKV-117, harvested from a single person, stood out for its anti-Zika action, the team reported in the journal Nature.
Antibodies are the footsoldiers in the body's defences.
They are Y-shaped proteins produced by the immune system in response to the presence of so-called antigens -- disease-causing pathogens.
Each type of antibody binds to a specific antigen, then inactivates it directly or tags it for destruction by other immune cells.
