Sidoarjo Regency (Indonesia: Kabupaten Sidoarjo) is a regency in East Java, Indonesia. It is bordered by Surabaya city and Gresik Regency to the north, by Pasuruan Regency to the south, by Mojokerto Regency to the west and by the Madura Strait to the east. It has an area of 714.24 km2, making it the smallest regency in East Java.As at the 2010 census, Sidoarjo Regency had a population of 2,191,489; while the 2020 census showed a small decline to 2,033,760, notably in the northern part of the regency immediately bordering Surabaya. The regency is part of the urban region surrounding Surabaya, known as 'Gerbangkertosusila' area.