| Afon Porth-llwyd | |
| Pushpin Map: | UK Wales |
| Pushpin Map Caption: | Location of the mouth in Wales |
| Subdivision Type1: | Country |
| Subdivision Name1: | Wales |
| Subdivision Type4: | District |
| Subdivision Name4: | Conwy |
| Source1: | Llyn Eigau |
| Mouth: | Confluence with Afon Conwy |
| Mouth Coordinates: | 53.1951°N -3.8351°W |
Afon Porth-llwyd is a river in Snowdonia in north-west Wales. It flows from Llyn Eigiau on the south-eastern edge of the Carneddau range to join the river Conwy.
Its waters are trapped at Coedty reservoir before flowing down to pass under Pont Newydd in Dolgarrog. From Coedty reservoir some water is also diverted to Dolgarrog Power station (along with water from Llyn Cowlyd) via large black pipes.