No, it isn’t considered to be an island even though it is separated from the mainland by water.  A bridge across the Corinth Canal provides a permanent link between the peninsula and mainland. However, Lefkas and Evia are called islands although they are also linked by bridges to the mainland. The difference seems to be that a man-made canal separates the Peloponnese whereas it’s the sea that separates both Lefkas and Evia.