The Welsh Language Society is likely to receive more than £500,000 from the will of a Welshman who spent most of his life in England.
Cymdeithas yr Iaith Gymraeg says the bequest by Howel Vaughan Lewis will secure its long-term financial future.
He left Ceredigion when he was a boy and died at 87 in 2005 after building an estimated £4m fortune in London.
The society said the money would put it on a firm financial footing for at least the next decade.
Cymdeithas' governing body has been meeting in Aberystwyth to discuss a shake-up of its campaigning and administrative structures after the windfall.
BBC Wales' Maniffesto programme, broadcast on S4C, has learned the extent of the donation.
"I don't think there's anything wrong in professionalising the society"
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, where he grew up.
Since it was formed in 1962 the society has largely relied on volunteers and member donations. It employs two full-time members of staff.
Cymdeithas told Maniffesto it would consider employing a full-time lobbyist at the Welsh assembly in an effort to professionalise the body, and recruit extra salaried officers in areas such as south-east Wales.
However, it is likely to reject calls to invest in new offices or headquarters or appoint a chief executive to run the society on a day-to-day basis.
The society confirmed that it had already received "some of the money" but was awaiting details of the full amount, which it hoped to get soon.
Chairman Steffan Cravos said: "I don't think there's anything wrong in professionalising the society - I think having a full-time lobbyist in the assembly is a good idea."
Maniffesto is on S4C at 1300 GMT on Sunday.
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