The present disclosure concerns aptamers of formula (I) capable of specifically binding to Factor XIa. The aptamers can be used to prevent, treat or alleviate the symptoms of thrombosis. The aptamers can also be used to detect Factor XIa in a sample and/or purify Factor XIa from a sample. The aptamers can further be used to identity putative therapeutic agents for the prevention, treatment or alleviation of symptoms associated with thrombosis.
