Male. A polar bear and the Captain of the Octonauts. He possesses immense strength, great physical and mental fortitude, and is the most skilled pilot of the GUPs. He carries an Octo-compass on his belt. He speaks with a navy-like discipline but has a warm heart; he has never once lost his temper even when the crew causes trouble. Despite his bravery, he suffers from claustrophobia due to an accident where he fell into an ice hole as a cub, though he partially overcame this after escaping a cenote. He is addressed as 'Captain'.

Male. A cat and the Lieutenant of the Octonauts. He is the strongest after Barnacles. Coming from a family of pirates, he often tells the crew various legends, though these tales usually exaggerate the traits of specific animals. He is fearless and bold but has a phobia of spiders, which he overcame after meeting sea spiders. He has a habit of ending his sentences with a soft, melodic tone and sometimes mispronounces animal names (e.g., calling a Siphonophore a 'Siphoni-pie-po-po'). He shouts "Yeow!" whenever he feels a thrill.

Male. An Adélie penguin and the Octonauts' medic. He is timid and very shy, but becomes braver than anyone when an animal is sick or injured. He is best friends with Kwazii. He is physically weak and is the smallest member after Tunip. Due to his gentle nature, he speaks more softly than the others. Although he looks like he isn't wearing anything, he actually wears clothes that are the exact same color and pattern as a penguin's body. When startled, he exclaims, "Flapping flippers!"

Female. A dachshund who serves as the IT officer and photographer for the Octonauts. She always carries her camera and treasures it dearly. She is energetic, positive, and kind. Her technical skills are top-tier, nearly rivaling Tweak's; while Tweak handles development and repairs, Dashi often handles test piloting, such as with the Octo-Max Suit. She often runs into trouble while out exploring with Shellington. She wears headphones when she sleeps.

Male. A sea otter and the Octonauts' marine biologist. He always carries a magnifying glass with incredible power—enough to see individual cells in coral reefs. While he has vast knowledge of marine life, he can be a bit clumsy. As he is not tech-savvy, his piloting skills used to be poor, though they have improved recently. Since he still isn't a master pilot, Dashi usually takes the wheel for him. Despite being a sea otter, he is allergic to sea urchins.

Female. A rabbit and the Octonauts' engineer. She loves carrots immensely—she supposedly ate a thousand carrots while building the Deep Sea Station. She wears a pink headband. She often plays console games in her room. When flustered or surprised, she twists her ears like a pretzel. Her catchphrases include "Crunchy carrots!" and "Faster than a K-Gup!" She is originally from the Everglades in Florida.

Male. A Dumbo octopus and the Octonauts' oceanographer and founder. He has a somewhat cynical personality. He doesn't go on missions often, but he always shows up when the Octo-alert sounds. Despite being a Dumbo octopus (which usually can't), he is able to squirt ink. He usually moves around the Octopod in a floating chair. Though not great with machines, he once managed a shaky manual override of the Octopod, a feat usually only Barnacles can perform. With his eight arms, he is unbeatable at ping-pong. He is addressed as 'Professor'.

Male. A 'Vegimal,' a creature that is half-tuna and half-turnip. He is yellow and serves as the Octonauts' cook. When scared, he does a 180-degree turn and faints. His specialty is 'Fish Biscuits,' fish-shaped crackers. He can cook almost anything but hates bitter food. He treasures the Vegibot, which he claims is his own invention. He speaks a mix of normal language and Vegimalese (e.g., "Cheepa, cheepa!"). He is yellow on top and turquoise on the bottom, with green leaves on his head.

Exploration assistant robots stationed at the Octo Deep Sea Station. There are green (Unit 3), light blue (Unit 2), and yellow (Unit 4) models. They have video cameras for eyes to transmit footage and pincers to lift objects like rocks. Their arms can extend, though they cannot lift extremely heavy items. Pressing the protrusion on their head shuts down the system. They do not speak.

A robot with a system similar to the Octobots but equipped with a cooking program. It looks very similar to Tunip. Consequently, it speaks like Tunip, saying "Cheepa, cheepa," albeit with a mechanical filter. It can also speak normally. Like the Octobots, pressing the protrusion on its head—designed to look like a leaf—shuts down the system.