Selection of aptamers is crucial iterative process which is significant for many applications from biosensors to biomedical applications. The process of selecting aptamers is called systematic evolution of ligands by exponential enrichment (SELEX). This process is costly, time consuming and labour-intensive. In this communication we present T_SELEX API together with its algorithms and application. This program is an autonomous robust python package specifically engineered for designing aptamers and screening of RNA aptamers against macromolecular targets. The tool is evaluated from generation of sequence library of RNA aptamers, multiscale folding or secondary structure predictions, multiscale tertiary structure predictions, virtual screening and detailed analysis. The application was tested by screening a generated aptamer library against HIV-1 protease. The source code of the T_SELEX tool is available on GitHub [https://github.com/CMCDD/T_SELEX] and can be used directly following instructions on the readme file.