Afon Seiont (Welsh, meaning 'River Seiont' in English) is a river in Gwynedd, Wales which runs into the Menai Strait.

Its source is the outflow of Llyn Padarn near Llanberis, and it flows out in a generally northwest direction. Between the outflow and the village of Llanrug it is known as the Afon Rhythallt, changing its name just after the village. Its mouth is in the town of Caernarfon, forming a natural harbour as it flows out into the Menai Strait. The Afon Nant Peris provides the main inflow into Llyn Peris which then drains into Llyn Padarn with the addition of the waters of the Afon Arddu which drains the northern slopes of Snowdon.

The name of the Roman fort of Segontium, near Caernarfon, is based on the Latinised form of the name 'Seiont'. In the Brythonic language *seg-ontio translates as 'strong place'.^[1]

• Aber Afon Seiont - Site of Special Scientific Interest at mouth of river
• List of rivers of Wales

v t e Gwynedd
Principal settlements	Bala Bangor Barmouth Bethesda Blaenau Ffestiniog Caernarfon Criccieth Dolgellau Ffestiniog Harlech Nefyn Porthmadog Pwllheli Tywyn
Towns and villages	Aberangell Aberdaron Aberdesach Aberdyfi Abererch Abergeirw Abergwyngregyn Abergynolwyn Aberllefenni Abersoch Abertrinant Afon Wen Anelog Arthog Beddgelert Bethania Bethel Bethesda Betws Garmon Bodferin Boduan Bontddu Bontnewydd Botwnnog Bryncroes Bryn-crug Brynrefail Buan Bwlch-derwin Caeathro Capel Celyn Carmel Carnguwch Ceidio Chwilog Clwt-y-bont Clynnog Fawr Corris Uchaf Corris Croesor Cwm-y-glo Deiniolen Dinas Dinas Dinlle Dinas Mawddwy Dinorwig Dolbenmaen Dolmelinllyn Dwygyfylchi Edern Efailnewydd Eisingrug Fairbourne Friog Frongoch Ganllwyd Garndolbenmaen Garneddwen Gellilydan Glasinfryn Groeslon Llan Ffestiniog Llanaber Llanaelhaearn Llanarmon Llanbedr Llanbedrog Llanberis Llandanwg Llanddeiniolen Llandegwning Llandudwen Llandwrog Llandygai Llanegryn Llanelltyd Llanengan Llanfaelrhys Llanfaglan Llanfair Llanfihangel Bachellaeth Llanfihangel-y-Pennant Llanfihangel-y-Pennant, Dolbenmaen Llanfrothen Llangelynnin Llangian Llangwnnadl Llangybi Llaniestyn Llanigian Llanllechid Llanllyfni Llannor Llanrug Llanuwchllyn Llanwnda Llanymawddwy Llanystumdwy Llithfaen Llwyndyrys Llwyngwril Maentwrog Mallwyd Mellteyrn Minffordd Morfa Bychan Morfa Nefyn Mynydd Llandygai Mynydd Nefyn Mynytho Nantlle Valley Nantmor Nasareth Nebo Pant Glas Pen-y-meinl Penffridd Penisa'r Waun Penllech Penllyn Penmaenpool Pennal Penrhos Penrhyndeudraeth Pentre Gwynfryn Penygroes Pistyll Pontrug Porthdinllaen Portmeirion Prenteg Rachub Rhiwddolion Rhos-y-gwaliau Rhosgadfan Rhoshirwaun Rhoslefair Rhostryfan Rhyd Ddu Rhyd Rhyd-uchaf Rhydyclafdy Sarn Meyllteyrn Soar Tal-y-llyn Talsarnau Tal-y-bont (near Bangor) Tal-y-bont (near Barmouth) Talysarn Tanygrisiau Trawsfynydd Trefor Tregarth Tremadog Tudweiliog Tywyn Waunfawr Y Felinheli Y Ffor Y Fron Y Rhiw
Education	Bangor University Coleg Harlech Coleg Meirion-Dwyfor Coleg Menai Schools
Castles and forts	Caernarfon Castle Castell y Bere Criccieth Castle Dolbadarn Castle Harlech Castle Fort Belan
Rivers	Afon Aber River Adda Afon Artro Afon Cegin Afon Cwmnantcol Afon Daron Afon Dwyfor Afon Dwyryd Afon Dysynni Afon Fathew Afon Glaslyn Afon Llyfni Afon Mawddach Afon Ogwen Afon Rhythallt Afon Seiont Afon Tryweryn
Islands	Bardsey Island Ynys Gifftan Ynys Gwylan-fawr Saint Tudwal's Islands
Topics	Parliamentary constituencies Churches Places Schools SSSIs Country houses Listed buildings Grade I Grade II* Registered parks and gardens History Lord Lieutenants High Sheriffs Museums Public art
Geography Wales

53°09′N 4°16′W / 53.150°N 4.267°W / 53.150; -4.267