Between July 2009 and June 2011, rotavirus was detected in 507 of 4597 episodes of acute gastroenteritis in children aged < 3 years in Gipuzkoa (Basque Country, Spain), of which the G-type was determined in 458 (90·3%). During the annual seasonal epidemic of 2010–2011, the unusual G-type 12 was predominant, causing 65% (145/223) of cases of rotavirus gastroenteritis. All the G12 strains were clustered in lineage III and were preferentially associated with P-type 8. This epidemic was characterized by broad geographical distribution (rural and urban) and, over 7 months, affected both infants and children, the most frequently affected being children between 4 and 24 months. Of children with rotavirus G12, 16% required hospital admission, the admission rate in children aged <2 years being 20·7 cases/10 000 children. The sudden emergence and predominance of G12 rotaviruses documented in this winter outbreak suggest that they may soon become a major human rotavirus genotype.