Prompt injection attacks are an emerging threat to large language models (LLMs), enabling malicious users to manipulate outputs through carefully designed inputs. Existing detection approaches often require centralizing prompt data, creating significant privacy risks. This paper proposes a privacy-preserving prompt injection detection framework based on federated learning and embedding-based classification. A curated dataset of benign and adversarial prompts was encoded with sentence embedding and used to train both centralized and federated logistic regression models. The federated approach preserved privacy by sharing only model parameters across clients, while achieving detection performance comparable to centralized training. Results demonstrate that effective prompt injection detection is feasible without exposing raw data, making this one of the first explorations of federated security for LLMs. Although the dataset is limited in scale, the findings establish a strong proof-of-concept and highlight new directions for building secure and privacy-aware LLM systems.