Platelets are well-known for their functions in blood clotting and vascular repair. However, in recent years, the regulatory role of platelets in the occurrence and development of malignant tumors has received significant attention. While extensive research has been conducted on the regulation of tumors by circulating platelets in peripheral blood, there is a lack of coherence and continuity among these studies. The tumor microenvironment encompasses the intricate network of cellular and acellular elements that surround and interact with tumor cells, creating a supportive ecosystem for their survival and growth. It plays a crucial role in the initiation and progression of tumors. Similar to dark matter in the universe, platelets, as tiny and enigmatic entities, play an essential role in tumor development and treatment within the tumor microenvironment. Although our current understanding of platelet regulation in the tumor microenvironment is limited, they hold immense untapped potential. In-depth studies on the tumor microenvironment have revealed platelets as a meaningful component, influencing various aspects of tumor development, metastasis, and immune evasion. Platelets, through the release of various bioactive substances or direct interaction with tumor cells, impact tumor progression while being influenced by the tumor in return. Therefore, understanding the role and mechanisms of platelets in the tumor microenvironment is of great importance for tumor prevention and treatment. This review provides a summary of the research progress on the interplay between platelets and tumors in the tumor microenvironment, and presents a promising outlook on the potential of platelets in tumor therapy.