A zoo representative told Dantri on November 6 that the release was carried out in cooperation with Cat Tien National Park.
The first group comprised four females and two males. The full programme involves 22 females and eight males.
The releases will take place in stages to allow conservationists to monitor how the deer adapt to their natural habitat before introducing additional animals.
According to the zoo, the hog deer is among the rare species it has successfully bred in captivity as part of its conservation efforts.
Cat Tien National Park is the first location selected to receive the animals for reintroduction.
"The release is intended to conserve the species, not because the zoo has too many deer," the representative said.
Before transportation, veterinarians sedated each deer, administered supplements and placed the animals in specially prepared transport crates to minimise stress and ensure their safety.
Cat Tien National Park is considered to have suitable habitat conditions for the reintroduction programme.
The hog deer (Axis porcinus) is classified as critically endangered on Vietnam's 2023 Red List.
It is also listed under Group IB of Circular 85 issued by the Ministry of Agriculture and Environment, which covers endangered, precious and rare forest animals subject to strict protection.