The landmark case, filed in 2014, has pitched Tran To Nga, a 79-year-old who claims she was a victim of Agent Orange, against 14 firms, including U.S. multinational companies Dow Chemical and Monsanto, now owned by German giant Bayer.
Tran To Nga confirmed to Reuters earlier media reports that the case had been thrown out. She added she would appeal against the ruling.
Tran, who worked as a journalist and activist in Vietnam in her 20s, has said she notably was suffering from Agent Orange effects, including type 2 diabetes and a rare insulin allergy.
U.S. forces used Agent Orange to defoliate Vietnamese jungles and to destroy Viet Cong crops during the war.
The legal proceedings could have been be the first to provide compensation to a Vietnamese victim.
So far, only military veterans from the U.S. and other countries involved in the war have won compensation. (Reporting by Dominique Vidalon and Yiming Woo; Editing by Sudip Kar-Gupta and Andrew Cawthorne)