Lughnasadh or Lughnasa (pronounced /ˈluːnəsə/, LOO-nə-sə; Irish: Lúnasa, /ˈl̪ˠuːn̪ˠəsˠə/; Scottish Gaelic: Lùnastal, [ˈl̪ˠu:nəsd̥əl̪ˠ]; Manx: Luanistyn, [ˈluanɪst̪ən]) is a Gaelic festival marking the beginning of the harvest season.

Historically, it was widely observed throughout Ireland, Scotland and the Isle of Man. Originally it was held on 1 August, or about halfway between the summer solstice and autumn equinox. However, over time the celebrations shifted to the Sundays nearest this date. Lughnasadh is one of the four Gaelic seasonal festivals, along with Samhain, Imbolc and Beltane. It corresponds to other European harvest festivals such as the Welsh Gŵyl Awst and the English Lammas.