All octopuses, cuttlefish, and some squid are venomous in that they are able to inflict a poisonous, bite, sting, or wound. However most of them are harmless to humans.
The exception is the blue-ringed octopus. They are found in reefs and tidal pools from Japan down to Australia. In fact, we had a blue-ringed octopus alert for our local beaches this summer here in South Australia. The beach was fenced off in the areas where they were found and identified, but parents have to be particularly careful to supervise what their children are playing with!
They are not an aggressive species. When disturbed, their instinct is to run away. Only when cornered, picked up or trodden on, will they first display their blue rings in an attempt to frighten off the aggressor (you!), then sting as a last resort. A sting will cause numbness and breathing difficulties and, if not treated, can be fatal in minutes.
Pretty, aren't they?
You can read the SA news article by clicking on the External Link.