Kostreši Šaški is located in the municipality Sunja, in Sisak-Moslavina County at an elevation of 107 meters above sea level and it covers the area of 3.52 km² (1.36 sq. miles). According to the 2011 census report, there were 71 inhabitants. It is mostly populated by Serbs (53%), but also Croats (43%) and Bosniaks (4%). Nearby places: Papići, Jasenovčani, Slovinci, Timarci, Čapljani, Gornja Meminska, Srednja Meminska, Pobrđani, Staza and Rausovac.