Hubbub, a UK charity, found that 95% of the 108 sweaters it analyzed -- taken from 11 high-street and online retailers -- were fully or partially made from plastic materials.
Acrylic was the most common plastic fiber found in these festive sweaters -- with 44% made entirely from the material and three-quarters containing acrylic to some degree.
A 2016 study by Plymouth University in the UK revealed that acrylic releases almost 730,000 microfibers in every wash, far more than polyester or polyester-cotton blends.
Given the high use of acrylic, Hubbub warns that an average Christmas sweater is "likely to add to the issue of plastic pollution in our oceans."
The charity estimates that customers in the UK will buy 12 million Christmas sweaters this festive season -- despite already owning 65 million -- and that two in five of these garments will be worn just once over the holiday period.
Sarah Divall, project co-ordinator at Hubbub, said: "Fast fashion is a major threat to the natural world and Christmas jumpers are particularly problematic as so many contain plastic."
Divall added that sweaters should be "for life, not just for Christmas."
To tackle the environmental issue, the charity recommends that people swap their sweaters with friends or buy second-hand.
The warning comes ahead of the charity Save the Children's annual Christmas Jumper Day on December 13, when people in the UK are encouraged to put on their festive wear and donate to charity.
Gemma Sherrington, executive director of marketing and fundraising at Save the Children UK, told CNN that the charity has been "working hard" to ensure the annual event happens "without exacerbating the enormous climate problems" the world faces.
Sherrington added that Save the Children wants to make this year's event "the most sustainable" and has encouraged offices and schools to hold sweater swaps.