We haven’t found any tradition in which using octopus as food be considered impermissible. However, we can infer this forbidden from other traditions (ahadith) in which the main rules have been stated in this regards. This means, any types of marine animals which haven’t scale are being declared unlawful (haram) according to these traditions.
It should be noted that octopus is belong to the above-mentioned fauna.[1] As a result, eating such that fauna (octopus) is haram.

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