The retailer said the move was being made to rescue the packaging impact on the environment.
The recyclable plastic bottles for the Special Dry Imperial own-brand will weigh less than a sixth of the current glass ones.
The Co-op said it would save 270 tonnes of glass a year and reduce the energy needed to transport its vodka.
The company has already taken steps to reduce the weight of its own-label wine bottles and introduced lighter glass for its ales last summer.
Vicky Wood, category marketing manager for beverages, said: “If the introduction of the 35cl plastic vodka bottle is successful, we will look to rolling PET packaging out to other 35cl spirit bottles, as well as extending it to other sizes.”
The initiative follows the introduction of the Co-op’s ethical policy on food last year.