Marathonisi island is also located in the Gulf of Laganas, across from the beach of Keri.  Its perimeter is approximately one mile and its shape resembles a turtle.  It can be reached by boat from both Keri coast and Agios Sostis port or if you prefer you can go by kayak or canoe. The island has a large white sandy beach which is where the Caretta-Caretta nest their eggs and a smaller beach which consists of small white pebbles.  The vegetation is thick with pine-trees and green oak trees and the crystal clear waters really are amazing.  Due to it being a nesting area for the Caretta-Caretta the island is protected by the National Marine Park.

The sandy beach is covered thinly by a relatively small number of plant types, including: Cakile maritima, Medigaco marina, Eryngium maritimum, Euphorbia peplis, Centaurea sonchifolia, Pancratium maritimum, Cynodon dactylon, Elymus repens, Cyperus capitatus.

Within the inner parts of the sandy beach you can find samples of Pistacia lentisc. In the internal area of Marathonisi the vegetation is: Quercus coccifera, Pistacia lentiscus, Phillyrea latifolia, Rhamnus alaternus, Calycotome villosa.

The abrupt rocks on the island's beach have: Ephedra campylopoda, Phlomis fruticosa, Ptilostemon chamaepeuce, Helichrysum stoechas, Dianthus fruticosus subsp. occidentalis, Limonium zacynthium, Dancus carota, Capparis spinosa