The rejection is a major setback for Údaras na Gaeltachta, the Gaeltacht development authority, which based its jobs growth strategy on employment the new marine park would create.
Páirc na Mara, which was to be located in Carna in the Iorras Aithneach area of the Gaeltacht, had attracted significant marine interest when proposals were initially rolled out.
The plan was welcomed by members of the Carna and Cill Chiaráin communities.
However, the initial planning application was turned down by Galway County Council in 2021.
Bord Pleanála has given several reasons for rejecting the appeal. It highlighted lack of information on potential impact of climate change, and impact on water levels and the water supply in sources nearby.
Udaras na Gaeltachta, has said it is awaiting a copy of the full inspector’s report before commenting further.