But experts have now revealed that this assumption is wrong - as two of their long tentacle-like limbs are in fact legs.
A study by scientists at Sea Life centres across Europe found that the invertebrates move across the sea bed using their two rearmost limbs, leaving the other six free for the important business of feeding.
Researchers who observed the creatures in action found they push off with the "legs" and then employ the other tentacles to pump themselves along.
The study, the largest of its type carried out, was designed to show if octopuses favoured one side or the other.
But it found that octopuses are ambidextrous, though many seem to favour their third arm from the front to eat with.
Octopuses either swim or crawl across the seabed.
If they need to move fast they can shoot a jet of water from an opening in their body.
Scientists from 20 centres across Europe analysed data from 2,000 observations of common octopuses to get the results.
Claire Little, a marine expert from the Weymouth Sea Life Centre in Dorset, said: "We've found that octopuses effectively have six arms and two legs.
"Observations showed that they use the rearmost two to get around over rocks and the seabed.
"They also use these two legs to push off when they wish to swim, and then other tentacles are used to propel them."