The largest peninsula in the Adriatic Sea is Istria, which is located in the northern part of the Adriatic Sea and is bordered by Croatia, Slovenia, and Italy. It is the largest peninsula in the Adriatic Sea, covering an area of 3, 160 square miles. Istria is known for its picturesque coastline, rolling hills, and ancient towns. It is also home to a variety of wildlife, including deer, wild boar, and birds.