A post-mortem of the creatures, found ashore near the town of Toenning in Schleswig-Holstein, Germany, showed their stomachs were full of plastic.
This plastic included a 13-metre-long (43-foot-long) fisherman’s net and a 70-centimetre (28-inch) piece of plastic from a car.
A man posted a shocking image on Instagram of some of the plastic found in their stomachs.
Scientists thought it was likely they perished from heart failure due to starvation.
“These findings show us the results of our plastic orientated society,” Schleswig-Holstein environment Minister Robert Habeck told the Daily Mail.
“Animals inadvertently consume plastic and plastic waste which causes them to suffer and at worst, causes them to starve with full stomachs.”
They all weighed around 15 tonnes – and the average weight of a sperm whale is 32 to 41 tonnes.
Experts believe storms in the northeast Atlantic shifted the whales’ food source into the North Sea.
The whales followed the food source and found themselves stranded in shallow water, where they starved to death.
This news comes after six dead sperm whales were found beached in Norfolk in February.
Six of the creatures died on a Norfolk beach in the space of just two weeks.
“Historically we have had mass strandings but nothing of this scale for decades.”
Scientists are investigating whether man-made pollution is to blame.