Sergio Villanueva Fernández (born 2 November 1975) is a Spanish professional footballer who plays for CD Lealtad as a midfielder.
Born in Mieres, Asturias, Villanueva graduated from Caudal Deportivo's youth setup, and made his senior debuts while on loan at amateurs Santiago de Aller CF in 1994. In 1996 he signed for Sporting de Gijón, being assigned to the reserves in Segunda División B but eventually returning to his first club in the following year, in a season-long loan deal.
Villanueva moved to another reserve team in the 1998 summer, Deportivo Alavés B from Tercera División. In January of the following year he joined SD Noja in the third level, and subsequently represented Club Marino de Luanco, playing in both the third and fourth tiers.
In July 2002 Villanueva signed with Segunda División's SD Eibar. His first match as a professional took place on 1 September, aged 27, playing the last two minutes in a 0–0 away home draw against Polideportivo Ejido. However, after featuring rarely, he moved to Zamora CF in January of the following year.
Villanueva joined third division side UD Vecindario in the 2005 summer, appearing regularly in his first season, which ended in promotion. He was played sparingly in his second, and terminated his contract on 22 January 2007, subsequently signing for Real Oviedo.
Villanueva moved back to former club Marino de Luanco in 2010, being promoted to the third level at the first attempt. In August 2012 he joined CD Lealtad, also in his native region and division four.