Biomarkers are signature molecules able to indicate specific physiological states of cells. Identification of reliable biomarkers is essential for early diagnosis and adaptive treatment of diseases, especially cancer. Aptamers are single-stranded oligonucleotides generated by an in vitro screening method called Systematic Evolution of Ligands by Exponential Enrichment (SELEX). They can recognize their cognate targets with selectivity and affinity comparable to protein antibodies. In addition, aptamers have superiorities including easy synthesis, high chemical stability, convenient modification and flexible design. As such, these DNA molecules show great promise as powerful molecular probes for biomarker discovery and biomarker-based clinical applications. Using complex samples as targets, a panel of aptamers can be systematically generated for comprehensive recognition of disease-specific proteins, which can potentially serve as biomarkers. This review describes the current methods for biomarker discovery using aptamers.