Abstract: Heavy metal (metalloid) pollution in aquatic systems is a critical global environmental concern, threatening the stability of aquatic ecosystems and posing severe risks to human health. Current engineering approaches for treating heavy metal (metalloid) pollution in wastewater are hindered by low efficiency, high costs, prolonged treatment times, and the potential to cause secondary pollution, further exacerbating ecosystem degradation. Consequently, the development of efficient, cost-effective, and environmentally sustainable solutions has become a priority. Furthermore, heavy metal (metalloid) contamination imposes immense pressure on freshwater ecosystems, underscoring the urgent need for efficient monitoring tools and scientific methods for evaluating their toxicity. Duckweed, a term encompassing various genera within the family Lemnaceae, including Lemna, Spirodela, and Wolffia, has emerged as a promising model plant in freshwater ecological research. Its rapid growth, ease of cultivation under controlled conditions, and sensitivity to environmental changes make it an ideal candidate for both early detection and subsequent remediation of heavy metal (metalloid) pollution. This review consolidates two decades of research on the application of duckweed as a bioindicator and remediation agent for heavy metal pollution in aquatic ecosystems. We summarize its response mechanisms and applications across molecular, cellular, and individual levels, providing insights into existing challenges and future development trends. This review aims to provide a theoretical framework for its strategic and effective application in monitoring and mitigating heavy metal (metalloid) pollution in freshwater ecosystems.