A group of fisherman in the Sacramento River had an unbelievable run-in when a massive humpback whale appeared in the waterway.

Denny Glover said he and his pals were on a fishing trip south of the Rio Vista Bridge Wednesday when one of them spotted spray from the water.

“I honestly, I didn’t believe it. I thought he had mistaken a dolphin or porpoise or whatever,” Glover told KCRA. “I just didn’t believe him.”

Glover said when the spray appeared a second time, there was no doubt “it was a whale.”

“Then we saw another spout and there’s no mistaking the size of a whale spout. When it blows when it comes up it’s a big mist of water,” he explained.

“Just the part of the whale that breached was longer than our 21-foot boat.”

The fisherman managed to capture video of the sighting with his phone, showing the whale swimming in the river delta.

It’s not the first time the humpback has been spotted off course. The giant whale has been identified by the Marine Mammal Center as CRC-11620 and named Rio, the center told the California Post.

Its movements have been studied by researchers in the area for more than two decades.

There’s been documented sightings of CRC-11620 15 times in the last 24 years as it traveled off the California and Mexico coast, with the first sighting in 2002 off the coast of California.

Earlier this week, it was spotted in the San Francisco Bay before making its way east into the Sacramento-San Joaquin River Delta.

The Marine Mammal Center confirmed the sighting, stressing the importance of the public to keep their distance.

“While it’s unknown whether this whale is just passing through or if there is an underlying, unknown ailment, the best thing people can do is experience this adult whale’s presence from a respectful distance,” Kathi George, Director of Cetacean Conservation Biology at The Marine Mammal Center, told the Post.

“This whale is in and out of normal habitat area. Our team is actively monitoring the situation, and we kindly ask the public to please avoid changing the whale’s course and let the animal decide where it wants to go.”

In 1985, a whale named Humphrey spent several weeks in the Rio Vista area, capturing national attention before making its way back to the ocean.

In 2007, mother and calf humpback whales — Delta and Dawn — excited observers in the area for 20 days before getting back on course.