Aptamers, which are short single-stranded DNA or RNA molecules, are often termed “chemical antibodies” because of their superior ability to bind to targets with higher specificity. This versatility and reproducibility of the aptamers, compared to conventional antibodies, make them highly suitable for detecting cancer biomarkers like prostate-specific antigen (PSA), human epidermal growth factor receptor 2 (HER2), and CA125. Moreover, they are gaining attention as a groundbreaking class of targeted therapeutics. Aptamer-based biosensors (aptasensors) have opened up transformative progress in the field of cancer diagnostics, offering unparalleled specificity, stability, and cost-effectiveness compared to conventional methods. This chapter explores how aptamers function as biorecognition elements in the detection of cancer biomarkers. Different types of aptasensors, ranging from optical and electrochemical models to those enhanced with nanomaterials were designed and combined with advanced technologies such as artificial intelligence, microfluidic systems, and innovative materials like graphene and gold nanoparticles (AuNPs). Real-world applications of aptasensors are illustrated through selected case studies and clinical examples, particularly in the early diagnosis and ongoing monitoring of breast, lung, and ovarian cancers. Aptamer-based biosensors offer significant potential in cancer diagnostics. This chapter also highlights key obstacles that need to be addressed—particularly issues related to sensitivity in complex biological environments, challenges in large-scale production, and existing regulatory barriers. Integrating aptasensors into wearable devices and tailoring them for personalized healthcare would lead to a promising direction for future clinical research. The chapter concludes with a forward-looking perspective on how these biosensors could transform current approaches to cancer detection and treatment.