Largest Sea Creatures that Ever Existed

There are some amazing large sea monsters that have inhabited our oceans. Here are the top 20 Largest Sea Creatures that Ever Existed!

September 21, 2022・

9 min read

The vast oceans of our little blue marble, which makes up 70% of the planet, are for the most part still unexplored. So much so, that new discoveries regularly pop up every now and then, slowly revealing both the wonders and horrors hiding in it.

It’s true: we have yet to find out even half of what the world’s deep waters hide, but the more we learn, the more it’ll keep us up at night thinking how little we humans are compared to what lurks at the bottom of the sea. Here are the 20 largest sea creatures that ever existed, ordered by weight and unusual features.

20. Thalattoarchon

Not a whole lot is known about the Thalattoarchon, considering it was only discovered in 2010. However, experts can place its size to be around 28 feet long and that it belonged to the Ichthyosaur family: a distinct kind of prehistoric marine reptile that lived during the Triassic period.

Based on fossils, it is possible that it resembled a dolphin save for its head, which is complete nightmare fuel.

19. Thalassomedon

If the Loch Ness monster was real, then cross your fingers that it isn’t a Thalassomedon. These carnivorous creatures from the Cretaceous period that belonged to the Pliosaur family grew up to 40 feet long.

They were known to be one of the fastest underwater killers of the era. That’s mainly because of their fins, which had a length of 7 feet each, allowing them to cover quite a distance with each paddle.

18. Giant Pacific Octopus

At up to 30 feet long, from head to the tip of its tentacles, and weighing anywhere around 130 to 165 lbs. (although there are heavily debated records that even purport they could weigh up to a whopping 0.3 tons), this is the largest octopus that exists today.

Aside from its size, the Giant Pacific Octopus is also known to be one of the longest living octopi out there, having a lifespan that’s anywhere between 3 and 5 years.

17. Nothosaurus

The Nothosaurus were notorious predators during the Triassic era, as they are known to have been blitz hunters, in that they preferred attacking swiftly and by surprise to capture their prey. When you think about it, it’s understandable that they’d make use of this tactic.

They’re relatively smaller than other creatures of that era, being 13 ft. long and weighing only about 0.15 tons. But size doesn’t matter when you have the element of surprise.

16. Giant Squid

To this day, not much is known about the giant squid, especially when you consider that we have only recently been able to capture a live specimen not more than two decades ago. All other evidence either comes from video footage or carcasses that wash ashore.

As such, the largest known specimen humans have found of this deep sea jumbo calamari stands at 59 feet and weighs about 0.9 tons. Did I wrote calamari? I meant Kraken!

15. Jaekelopterus

The Jaekelopterus is the kind of creature that you can usually find in science fiction stories. At its simplest, it’s an ancient sea scorpion that lived nearly 400 million years ago. With a length reaching up to 9 feet, the Jaekelopterus has become the largest arthropod to have ever existed.

As for its weight, there are reports that place its weight somewhere around 0.36 tons, but it’s still a heavily discussed point among experts.

14. Tanystropheus

At 20 feet long and with a weight clocking around at only 0.4 tons, the Tanystropheus doesn’t look like it’s much of a threat compared to the other creatures on this list. That is, until you find out that it’s the kind of marine reptile that was amphibious.

That means it can hunt on both land and underwater. According to paleontologists, it lived during the Triassic era, preferring to prowl the depths than the lands for prey.

13. Giant Oarfish

Oarfish are a pretty rare sight in the world today, which gave way to the superstition that there’s a disaster coming when they wash ashore. True or not, the Giant Oarfish is a majestic sight indeed.

These bony fish can grow up to lengths of 36 feet, with the largest recorded weight being more than half a ton at around 600 kilograms.

12. Liopleurodon

The Liopleurodon is the perfect poster boy for nightmares that could be found in the oceans prehistoric or otherwise. Being a plesiosaur, which is a family of marine reptiles that are distinct from dinosaurs, this monster of the deep from the Jurassic period is 30 feet long and weighs about 1.75 tons.

If its size isn’t enough to make you wet your pants, then how about the fact that its mouth is lined with teeth the size of kitchen knives?

11. Dakosaurus

This monster from the Jurassic period is believed by experts to be similar to modern-day crocodiles. However, instead of legs, it has fins on its sides that allow it to glide underwater as it hunted down its prey.

Weighing two tons and growing to 16 feet long, the Dakosaurus’ most dangerous weapon was its teeth, which looked like rows of talons. As such, it was one of the deadliest predators of its age.

10. Edestus

The Edestus is a prehistoric shark that lived around 300 million years ago, in an era known as the Carboniferus period. Based on its remains, the Edestus is estimated to have grown up to 22 feet long, with an approximate weight of 2 tons.

Adding to its fearsome size, its nickname “scissor-toothed shark” just makes it all the more horrifying compared to the modern sharks of today.

9. Shastasaurus

The Shastasaurus is another one of the Triassic period’s giant marine monsters. This 65-foot-long reptile belonged to the Ichthyosaur family and weighed somewhere around 5 tons. Despite its imposing size, however, this breed of ancient animal was not exactly what you can call an apex predator, since it preferred to feed on small fish than hunt bigger game.

8. Mosasaurus

If you've seen Jurassic world, you'll recognize this creature. Though, back when it was alive, it was a surface dweller, rather than a performing monkey at an extremely dangerous amusement park. This behemoth lived during the Cretaceous period and is a true apex predator when you consider its 60-foot long body that weighs somewhere within the area of 5-6 tons.

Also, just because it’s big doesn’t mean it’s sluggish: the giant fins on the Mosasaurus made it one of the most efficient hunters during its era, and with a mouth like that, it can kill with a single chomp.

7. Tylosaurus

The Tylosaurus is what you could call a close cousin of the Mosasaurus. They are, on the other hand, smaller compared to their counterparts, being that they grow to only 50 feet long. As for their weight, it’s a heavily debated topic with theories ranging from anywhere between 15-20 tons.

What they lack in size, though, they make up for in total savagery. Even by Cretaceous era standards, the Tylosaurus is one bad motherflipper: it’s the kind of monster that, according to its remains, regularly fed on its young.

6. Whale Shark

Whale sharks, if anything, hold the right to bear the title of “gentle giant.” This krill-eating species is the largest known to man, with lengths reaching up to 41.5 feet and weighing approximately 20 tons.

Sadly, Whale Sharks are in danger of becoming extinct thanks to a variety of reasons. Its population is being depleted because of overhunting by humans for its fins and meat: a sad fact that makes you question who the monsters really are.

5. Sperm Whale

Sperm whales can grow up to lengths of 67 feet and can weigh approximately 45 tons. That being the case, they are known as one of the largest predators in the known world. To be specific, they’re recorded as the largest toothed predators bar none.

However, despite their sizes having the capacity to kill a man with one swish of their tails, Sperm Whales actually prefer minding their business in the deep sea, hunting mostly squid.

4. Basilosaurus

Despite its name suggesting that it’s a reptile, the Basilosaurus is actually an ancestor of the modern-day whale. However, by no means can you call it a “gentle giant.”

This titan lived about 40 million years ago and was known to be a vicious predator, eating sharks like they’re a bunch of guppies. According to estimates, they grew up to 60 feet long and weighed up to 60 tons.

3. Fin Whale

Fin whales are known as the second-largest animal in the world, which is just right behind the Blue Whale – another entry on this list – in terms of size. These marine mammals can grow up to 89 feet long and possess a weight of 74 tons.

Like other whales, this species mostly feeds on small fish and crustaceans by way of filter-feeding. Today, Fin Whales still belong on the endangered species list because of overhunting by man back in the 1900s.

2. Blue Whale

Blue whales are the largest animals known to man. Standing at around 90 feet long, with a weight reaching up to 200 tons, these aquatic marvels are also becoming rarer and rarer in the world today.

Despite Blue Whales not being predators, since their diet consists mostly of krill (tiny shrimp that it sucks into its mouth), there’s still no reason to think that these giants cannot swallow you whole in one big gulp if you get in their way.

To put its size into perspective, here are two fascinating facts: The tongue of the Blue Whale alone weighs as much as an elephant and the heart as much as an automobile.

1. Megalodon

Despite being slightly smaller than the blue whale, its maximum size is still heavily debated, and given its ferociousness, you'd probably be frightened into thinking it was bigger than any other creature in this article.

The Megalodon is the kind of beast that scoffs at dinosaurs. In general terms, it’s a shark. Now, although sharks are scary enough as it is, just imagine one that looks almost exactly like a Great White, except it is 60 feet long and weighs anywhere between 75 to 100 tons.

To put things in perspective, Great White Sharks only grow somewhere around 20 feet and 2-3 tons. That is to say, the Megalodon just proves how immensely more brutal the world was a couple of million years ago.

If you were amazed at these incredibly large sea creatures, you might want to read about creatures more terrifying than megalodon living in the Mariana trench. Thanks for reading!