Five jobs are to be created by The Welsh Language Society using a £500,000 bequest from a Welshman who lived most of his life in England.
Llanwenog church also received money from Mr Lewis's estate
Cymdeithas yr Iaith Gymraeg was left the money by Howel Vaughan Lewis who died aged 87 in 2005.
At its annual general meeting in Aberystwyth on Saturday, the society voted for the money to be used to fund the jobs to enhance its work.
Society official Dafydd Morgan Lewis said the money was a "godsend".
He said it meant the society could invest in the future.
"We don't know how much we've got yet. But it's over £500,000.
"It's a boost for us. Since I can remember we have lived from hand to mouth.
"This at last will give us the opportunity to become a more professional campaigning group.
Another £100,000 of Mr Lewis' bequest will also be used to fund special projects within the society.
He left Alltyblaca in Ceredigion when he was a boy and died at 87 in 2005 after building an estimated £2.7m fortune in London.
After Mr Lewis died in March 2005 it emerged that he had left his entire estate to groups in Wales, including Plaid Cymru, Welsh youth movement Urdd Gobaith Cymru, Merched y Wawr (a Welsh language women's group) and St Gwenog's church at Llanwenog in Ceredigion.
Another society official Selwyn Jones said: "We're going to invest this money but as a society we're not going to change.
"We're going to continue to be a radical movement which believes that things in Wales must change."