Megan Fitzpatrick, a final year geography student, undertook the field research for her dissertation project.
She described the results of her work as "very shocking" and warned about the dangers facing humans who eat Sussex shellfish.
Megan carried out her investigation at Piddinghoe in the Lewes district.
The student examined the mussels along with 20 litres of brackish water, finding that microplastics were regularly ingested into the digestive tract and cells of the mussels.
Each mussel was found to have absorbed 51.6 particles of plastic per day, while there were 29.45 particles per litre of water.
Megan said: "The high concentrations of microplastics observed within the Sussex mussels was very shocking.
"Studies have suggested that these particles are causing harm to the mussels, however we are unaware of the full impacts as well as the potentially severe implications for shellfish eaters – including humans.
"Possible impacts for mussels may mirror the strangulation and entanglement seen within larger marine species and dolphins, now a commonly recognised image due to the influence of Blue Planet II."
Megan added that her motivation for this project stems from her love for the sea, which was instilled in her from a young age. She fears that human neglect of the natural world could have dire consequences in the future.
"The environment is treated as a rubbish bin by many, and if this high level of disrespect towards it continues, future generations will not be able to enjoy its benefits."
Megan will graduate in August.