Llyn Melynllyn

{{Use dmy dates|date=November 2019}}

{{Infobox body of water

| name = Llyn Melynllyn

| image = Melynllyn Reservoir - geograph.org.uk - 84864.jpg

| caption =

| image_bathymetry =

|pushpin_map=Wales Conwy

| caption_bathymetry =

| location = Snowdonia, North Wales

| coords = {{coord|53|10|20|N|3|56|39|W|type:waterbody_region:GB|display=inline,title}}

| type = natural, reservoir

| inflow =

| outflow = Afon Melynllyn

| catchment =

| basin_countries = United Kingdom

| length =

| width =

| area = {{convert|18.5|acre|ha|abbr=on}}

| depth =

| max-depth =

| volume =

| residence_time =

| shore =

| elevation = {{convert|2000|ft|m|-2|abbr=on}}

| islands =

| cities =

| frozen =

}}

{{langnf|cy|Llyn Melynllyn|yellow lake|italic=no}} is a lake within the Carneddau range of mountains in Snowdonia, North Wales.

It lies at a height of just over {{convert|2000|ft|m}}, and has an area of some {{convert|18.5|acre|m2}}.

Cliffs rise steeply from its western edge, up to the summit of Foel Grach, and down from which most of its feeder streams flow.

A small dam was built at its northern end in 1887, but this was deliberately breached in 1970. The lake acts as a reservoir for the Llandudno area.

Less than a kilometre to its north lies the larger Llyn Dulyn.

The outflow from the lake is called Afon Melynllyn, this stream flowing north-east to join Afon Dulyn, itself a tributary of the river Conwy.