The Brecon Beacons National Park Authority has refused permission for a major development of a former Ministry of Defence site at Cwrt y Gollen near Crickhowell.
The proposal involved 200 homes within the national park plus industrial premises, a care home and nursery facilities.
The decision was welcomed by the Campaign for National Parks which said it would have totally changed the culture and character of the area.
"It is reassuring to learn that members of the Planning Committee have listened carefully to the objections to this development and acted to protect the special, unique features of the Brecon Beacons for public enjoyment," said CNP head of Welsh affairs Norma Barry.
