The Woodland Trust wants to buy the land at Llennyrch woodland.
Natural Resources Wales (NRW) has given £50,000 but a further £750,000 is needed and a campaign will be launched on Tuesday at the National Eisteddfod in Meifod, Powys.
The charity said the area has been called a "Celtic rainforest" and it wants to improve wildlife on the site.
NRW chief executive Dr Emyr Roberts said: "This is a fantastic opportunity to bring the whole woodland area under conservation management."
The total cost of the project is £1.5m and the rest of the cost will be met by money left to the Woodland Trust.
A conservation charity is trying to raise £800,000 to purchase 550 acres of land in Snowdonia National Park.