The use of alternative feeds in dairy goat farming in the Mediterranean region could be a very promising solution to ensure the sustainability of this activity. These feeds can not only reduce production costs, but also improve product quality and market value. The objective of this study is to compare the quality of yogurt obtained from milk of dairy goats fed with oat hay (group C) or Posidonia oceanica seagrass debris (group E). Results showed that yogurt samples from group E had higher fat content and viscosity (P < 0.05) than the control group, whereas protein and lactose levels, as well as pH and acidity, were similar for both batches. A significant richness (P < 0.05) in total polyphenols, flavonoids, antioxidant agents DPPH, ABTS and ferric reducing antioxidant power, was observed in yogurt samples of group E. No differences were observed for total lactic microflora and L. delbrueckii subsp. Bulgaricus counts, but a significant increase of S. thermophilus count was detected (P < 0.05) when goats were fed Posidonia oceanica. The sensory analysis showed that yogurt from group E had a significant improved quality, which was reflected by higher scores for odor, flavor, texture and overall acceptability than group C. These findings reveal the positive role of feeding Posidonia oceanica to dairy goats and the remarkable improvements it has made to the nutritional properties and flavor of yogurt.