Charles had earlier said “decisive” action must be taken to save the world’s marine life and oceans.
He placed the female loggerhead turtle on to the golden sand at a beach in Malta after warning that the threat to the planet's marine ecology has reached a critical point where plastics are "now on the menu".
Prince Charles watched as the turtle, named Denise and estimated to be 18 years old, made her way into the water at Golden Bay, after recovering from breathing problems caused by plastic she had eaten.
Standing on the beach in his tailored suit and shirt and tie, Charles also helped to release two other loggerheads, also cared for by the Nature Trust Malta, which had both been snagged by fishing hooks.
He first picked an eight-year-old turtle named Tomisina, who had had a fishing hook removed from her throat - probably caused by trying to eat fish used for bait - and then helped Kirby, aged 15, who had a hook removed from a flipper.
When Kirby moved slowly along the sands and stopped, the Prince joked: "As long as it doesn't turn turtle - it's so marvellous."
Speaking at an ocean conservation conference, the Prince highlighted how plastics were increasingly found in fish caught for the dinner table.
In his keynote address, he told delegates at the Our Ocean conference in St Julian's: "As many of you know so well, the eight million tonnes of plastic that enter the sea every year - through our own doing I might add - is now almost ubiquitous.
"For all the plastic that we have produced since the 1950s that has ended up in the ocean is still with us in one form or another, so that wherever you swim there are particles of plastic near you and we are very close to reaching the point when whatever wild-caught fish you eat will contain plastic.
"Plastic is indeed now on the menu."