Neuendorf is a municipality in the district of Gäu in the canton of Solothurn in Switzerland. Neuendorf is first mentioned in 1417 as Núwen Dorff. …Neuendorf is a municipality in the district of Gäu in the canton of Solothurn in Switzerland. Neuendorf is first mentioned in 1417 as Núwen Dorff. Neuendorf has an area, as of 2009, of 7.16 square kilometers. Of this area, 3.51 km² or 49.0% is used for agricultural purposes, while 2.37 km² or 33.1% is forested. Of the rest of the land, 1.24 km² or 17.3% is settled, 0.01 km² or 0.1% is either rivers or lakes.