Reinforcement Learning from Human Feedback (RLHF) has been shown to effectively align large language models (LLMs) with human knowledge. However, the lack of human preference labels remains a significant bottleneck when applying RLHF to a downstream domain. Humans in RLHF play a critical role in injecting reasoning preferences into LLM, and we assume the reasoning process underlying human assessments may potentially be replaced by reasoning pathways derived from Knowledge Graphs (KGs). Inspired by this assumption, we propose Reinforcement Learning from Knowledge Graph Feedback (RLKGF), a novel method that leverages KG semantics and structure to derive RL rewards in the absence of manual annotations. Unlike Reinforcement Learning from AI Feedback (RLAIF), RLKGF directly integrates human priors encoded in KGs as the reward model, aligning LLM responses with expert knowledge without additional preference labeling or reward model training. RLKGF structures context-relevant facts into knowledge subgraphs and defines rewards by simulating information flow across semantic and logical connections between question and candidate response entities. Experiments on three public and one private medical dialogue dataset demonstrate that RLKGF significantly outperforms the competitive RLAIF in improving LLM diagnostic accuracy. The code is available at https://github.com/YanPioneer/RLKGF.