The illicit use of anabolic androgenic steroids (AAS) as performance-enhancing drugs remains a global issue threatening not only the credibility of competitive sports but also public health due to the well-documented adverse effects they elicit. AAS abuse is not restricted only to professional sports, but also extends to recreational athletes and adolescents as well as in livestock production as growth-promoting agents. Testosterone and nandrolone are among the AAS most frequently exploited. Gas chromatography-mass spectrometry is the reference method for AAS detection, but it is strictly laboratory-based and cannot be performed on-site. The great potential of aptamers in bioanalytical applications and specifically for the development of simple analytical tools suitable for on-site analysis has been extensively documented. In this report, we describe the selection and identification of aptamers binding nandrolone, exhibiting affinity dissociation constants in the low nanomolar range. A label-free colorimetric assay based on gold nanoparticles was developed using one of these novel aptamers for the detection of nandrolone and/or its metabolites. The assay could be deployed for the rapid, on-site, facile and cost-effective screening of samples and provide qualitative visual results with a red to purple/blue color change being indicative of a positive result.