Cath Palug (also Cath Paluc, Cath Balug, Cath Balwg, literally 'Palug's Cat') was a monstrous cat in Welsh mythology associated with Arthurian legend. Given birth to in Gwynedd by the pig Henwen of Cornwall, the cat was to haunt the Isle of Anglesey until Kay went to the island to hunt it down. Outside of Wales, the cat's opponent has been transposed to King Arthur himself or, occasionally, other legendary heroes such as Ogier the Dane. Cath Palug's name in French literature is Chapalu (Old French and variant modern forms: Capalu, Capalus).

The Welsh name Cat Palug may mean "scratching cat", but this is just one of a range of possible meanings. The word palug (paluc) is theorized to have a common pal- stem, which may mean: 'hit, strike', 'cut, lop', 'scratch, claw', or even 'dig, pierce'.

Chapalu, the French form can be broken down into chat 'cat' + palu 'bog', hence 'the bog cat'. In an Anglo-Norman poem (see §Li Romanz des Franceis), Chapalu and palu are connected in the story (the words are end-rhymed in the couplet).

Cath Palug is always localised near water, such as the lakes of Lac du Bourget and Lake Geneva in France (cf. also § Local lore), or the sea in Wales. One story describes it as some sort of fish-cat. The monstrous cat of Lausanne, which was the analogue in the Vulgate Merlin started out as a black kitten caught by a fisherman in his net.

Cath Palug is mentioned in just two works among early Welsh sources, the triads and a fragmentary poem.

Cath Palug's birth origins are given in "The Powerful Swineherds" in the Welsh Triads (Trioedd Ynys Prydein, end of the 13th century). According to this source, it started life as a kitten (lit "whelp"), given birth by the great white sow Henwen at the black rock in Llanfair [cy]. There the kitten was cast into the sea, but it crossed the Menai Strait and was found on Ynys Môn (Anglesey), where the sons of Palug raised it, not realizing the cat was to become one of the three great plagues of the island.

Cath Palug was fought and slain by Cai (Kay), or so it is implied, in the incomplete Old Welsh poem Pa Gur yv y Porthaur found in the Llyfr Du Caerfyrddin (The Black Book of Carmarthen, written before 1250). Kay had gone to destroy lleown (possibly meaning 'lions') in Môn (Anglesey). In the encounter, nine scores (180) warriors have been killed by the cat.

The fragmentary poem states that Kay's shield is mynud against the cat, which has been construed in various ways, but plausibly interpreted as "polished against Palug's cat". This description coincides with the Middle English story in the Lambeth manuscript, in which Arthur raises a shield (presumably mirrored) causing the cats to attack their own shadows reflected in it.