RankGPT is a Python toolkit specifically crafted to delve into the application of generative Large Language Models (LLMs), such as ChatGPT and GPT-4, for the purpose of relevance ranking within Information Retrieval (IR). It presents innovative techniques, including instructional permutation generation and a sliding window strategy, which help LLMs to efficiently rerank documents. Supporting a diverse array of LLMs—including GPT-3.5, GPT-4, Claude, Cohere, and Llama2 through LiteLLM—RankGPT offers comprehensive modules for retrieval, reranking, evaluation, and response analysis, thereby streamlining end-to-end processes. Additionally, the toolkit features a module dedicated to the in-depth analysis of input prompts and LLM outputs, effectively tackling reliability issues associated with LLM APIs and the non-deterministic nature of Mixture-of-Experts (MoE) models. Furthermore, it is designed to work with multiple backends, such as SGLang and TensorRT-LLM, making it compatible with a broad spectrum of LLMs. Among its resources, RankGPT's Model Zoo showcases various models, including LiT5 and MonoT5, which are conveniently hosted on Hugging Face, allowing users to easily access and implement them in their projects. Overall, RankGPT serves as a versatile and powerful toolkit for researchers and developers aiming to enhance the effectiveness of information retrieval systems through advanced LLM techniques.