We know that octopuses and many other animals are intelligent. We don't know for certain if anything nonhuman is sentient. Scientists and activist groups have been designing frameworks for sentience and tests to measure it. This paper [1] (about crabs, but discusses octopuses) has a very interesting section talking about defining and measuring sentience. It was used to support a recent bill in the UK [2] attempting t…
You are mixing up terms. All animals are sentient. Not all animals are intelligent. Only very few animals appear to be sapient. Octopuses are certainly one of the few animals that appear to be beyond a reasonable doubt.