The scale of contamination varied significantly across locations however, and the researchers warned against "alarmist" interpretations.
"We have to accept that almost everything, whether it's table salt or beverages, people are finding plastics in all kinds of different sources," said co-author Amanda Ford, a senior lecturer at the University of the South Pacific.
The research examined nearly 900 fish from 138 species -- all of them consumed locally -- taken from waters off Fiji, Tonga, Tuvalu and Vanuatu.
Overall, just under a third contained some microplastics, less than the global average of 49 percent.
But almost 75 percent of fish samples from Fiji's waters were affected, while just five percent were in Vanuatu.
Ford said the low levels there were "surprising" and potentially the result of different waste practices or ocean currents.
Pacific nations may be particularly vulnerable to microplastic pollution given limited waste management systems and rapid urban growth on some islands, the study said.
And the findings, published in the PLOS One journal, are especially relevant given most Pacific communities rely on fish as a key source of nutrition and livelihood.
Still, Ford warned "it's important we're not alarmist with this."
The findings are simply "evidence that plastic that ends up in the ocean breaks down into smaller pieces -- it can get into food," she told AFP.
The health implications of consuming microplastics remain unclear, especially at the low levels of contamination seen in many of the samples.
"We can't draw clear conclusions yet on the risks," she said.
A series of headline-grabbing studies in the last few years have reported detecting microplastics throughout human bodies -- inside blood, organs and even brains.
However, some of this research has been criticised recently by scientists warning the results could be detecting plastic from laboratories or confusing human tissue with plastic.
No one disputes however that these mostly invisible pieces of plastic are ubiquitous throughout the environment -- they have been found everywhere from the tops of mountains to the bottom of oceans.
The volume of plastic in the ocean is estimated to be anywhere between 75 and 199 million tonnes, according to the United Nations Environment Programme.
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