Metastasis is the main cause of failure in anticancer therapies, and is frequently related to poor prognosis for patients. The true challenge in extending cancer patient life expectancy, eventually managing cancer as a chronic disease with periodic but controllable relapses, relies on the development of effective therapeutic strategies specifically targeting key mechanisms involved in the metastatic cascade. Traditional chemotherapy with alkylating agents, microtubule inhibitors, and antimetabolites has shown limited efficacy against metastatic cells, largely due to the emergence of chemoresistant populations that undergo epithelial-to-mesenchymal transition (EMT), promoting the colonization of distant organs and sustaining metastatic progression. This scenario has spurred significant efforts to identify small molecules and biologics capable of interfering with specific steps in the metastatic process. In this review, we provide an overview of recent advances involving small interfering RNAs (siRNAs) and microRNAs (miRNAs) in cancer therapy. Although most of these agents are still under investigation and have not yet been approved for clinical use, insights into their development stage offer valuable information to identify new targets in the ongoing fight against metastasis. Particular emphasis is placed on the role of chemical modifications applied to siRNAs, such as backbone, sugar, terminal, base, and conjugation changes, and how these factors influence their stability, immunogenicity, and targeting precision. By integrating these aspects into the discussion, this review provides a focused and up-to-date resource for researchers in medicinal chemistry, drug delivery, and pharmaceutical formulation, where molecular design plays a critical role in therapeutic success.