MarkLLM is an open-source toolkit aimed at facilitating the research and application of watermarking technology in large language models (LLMs). As LLMs become increasingly prevalent, ensuring the authenticity and origin of machine-generated text becomes paramount. MarkLLM streamlines access, understanding, and evaluation of watermarking techniques by providing a unified and extensible platform. It supports various watermarking algorithms, including those from the KGW and EXP families, and offers visualization tools and evaluation modules to assist researchers and developers in assessing the detectability, robustness, and impact on text quality of watermarking techniques.