The same toxic algae that’s killed at least nine dogs across the country this summer is in full bloom in the water at popular city parks — and parents and pet owners are being warned to stay away.

Central Park’s Harlem Meer and Brooklyn’s Prospect Park Lake are coated with the blue-green muck, which if ingested can sicken kids and be fatal to dogs.

The water at Prospect Park Lake tested positive for “widespread high toxins” on Aug. 11, according to the state Department of Environmental Conservation, prompting the city to put up signs urging park users to “keep children and animals away from any blooms or surface scum.”

High toxins were also detected at Harlem Meer, where the quantity of algae, which can be laden with the cyanobacteria germ, was listed as having a “large localized” presence there.

That’s the second-highest designation, after “widespread.”

In addition, The Lake in Central Park had evidence of suspicious harmful algal booms, known as HABs, says the DEC, which found the algae in Central Park’s Turtle Pond on Aug. 11 and in Morningside Pond on Aug. 20.

Dog walkers in Harlem Meer held leashes extra tight while strolling near the glop-covered lake.

“That water is horrible –there is a ton of junk inside there plus the algae,” said Eric Quick, 30, as he walked his two dogs, Elvis, a pomeranian and Maltese mix and Mars, a poodle-terrier, around Harlem Meer on Thursday.

“There are a bunch of fish and turtles that die on a regular basis, so you don’t know what’s in there. You see a lot of floating turtles. It’s pretty awful.”

Sunday Humphrey, 53, said she doesn’t allow her pitbull-rottweiler mix, Bear, anywhere near the water.

“He used to wade over there in the sand and eat the grass along the Meer, but I don’t let him do that anymore,” she said.

Not all algae is harmful, but some has cyanobacteria, which killed three dogs who took a dip in a toxic Wilmington, North Carolina, pond on August 8.

It’s also linked to dog deaths in Texas, Georgia and Vermont this summer.

The Parks Department does not treat the algae, which grows in the summer before dying off during cooler weather. It’s thrived this year amid record high temperatures.

Quick said he hopes the city will do something to reduce the slimy mess.

“It would be great if they could clean it up,” he said. “Especially since there are a lot of untrained dogs who jump into this lake.”