The Swellies or Swillies () is an area of treacherous water in the Menai Strait, Wales, most commonly referring to the stretch between the Britannia Bridge and the Menai Bridge.
It is notable for its difficulty in safely navigating its shoals and rocks due to the whirlpools and surges that are the result of the tides washing around the island of Anglesey at different speeds. There are several small islands in The Swellies, the largest of which are Church Island () and (Red Weir Island in English but also known as Whitebait Island).
The Welsh name (variously spelled as "Pwll Ceris", "Pwll Cerys" or "Cerys bwll"), has been in popular usage since at least the sixteenth century when it became the subject for a number of poetic works by Welsh bards. Some of these works were reprinted in The Cambro-Briton journal in 1806, including works by the notable bards Gruffudd Hiraethog and Huw Roberts Len. These works would again be republished throughout the nineteenth century, including as part of thirteen englynion on Pwll Ceris published in the journal Y Grael in 1820. In 1804, Richard Llwyd published another well known Englyn "On Pwll Ceris, the Vortex in the Menai"
A medieval document quoted in the 2003 book The Menai Strait (translated from the 2003 Welsh original ) by Gwyn Pari Huws and Terry Beggs (Gwasg Gomer Press) states: "In that arm of the see that departeth between this island Mon and North Wales is a swelowe that draweth to schippes that seileth and sweloweth hem yn, as doth Scylla and Charybdis â therefore we may nouzt seile by this swalowe but slily at the full see."