The mysterious visitor showed up unexpectedly during an angling session posted to a fishing YouTube channel.
Something large and unexplained has been lurking near the surface of the Han River - a major stretch of water that flows through the South Korean capital city of Seoul.
The strange encounter was captured on camera during the recording of a recent episode of the fishing-themed YouTube channel - Jungdam Story.
The video, entitled "A Real Monster Emerges at the Han River in Seoul", seemed to start off like any ordinary fishing trip, but then, just after the 13-minute mark, a dark shape suddenly appeared in the water a short distance away.
As it got closer, the video's star got up and walked over to the water's edge to get a closer look.
Perplexed, he eventually picked up the camera and zoomed in to film the creature.
"What is that? I'm scared. It's moving toward me," he said.
He later noted that the creature had remained in the area for around 20 minutes and had been an estimated 10 meters in length. He also suggested that there may have been more than one.
"I've been fishing for decades, but I've never seen anything like this before," he said.
You can check out the sighting in the video below (starts at around 13:00).