GS-Hider is a steganography framework designed for 3D Gaussian Splatting (3DGS) to hide messages in 3D scenes and images without affecting rendering quality or security. The framework replaces the spherical harmonics coefficients in 3DGS with a coupled secured feature attribute, enabling the embedding of messages into the 3DGS point cloud files. A scene decoder and a message decoder are used to separate the original scene and hidden message. The framework ensures high fidelity, robustness, and capacity for hiding multiple 3D scenes or images. Extensive experiments show that GS-Hider can effectively conceal multimodal messages without compromising rendering quality and possesses exceptional security, robustness, capacity, and flexibility. The framework is available at https://xuanyuzhang21.github.io/project/gshider/. GS-Hider is the first method to study 3DGS steganography, offering applications in encrypted transmission, 3D compression, and copyright protection. The framework can hide an image, one or multiple 3D scenes in a single 3D scene, and is secure against steganography detection. It is also robust to degradation and can hide multiple scenes in a single scene without interference. The framework is designed to be flexible and efficient, with high rendering speed and fidelity.GS-Hider is a steganography framework designed for 3D Gaussian Splatting (3DGS) to hide messages in 3D scenes and images without affecting rendering quality or security. The framework replaces the spherical harmonics coefficients in 3DGS with a coupled secured feature attribute, enabling the embedding of messages into the 3DGS point cloud files. A scene decoder and a message decoder are used to separate the original scene and hidden message. The framework ensures high fidelity, robustness, and capacity for hiding multiple 3D scenes or images. Extensive experiments show that GS-Hider can effectively conceal multimodal messages without compromising rendering quality and possesses exceptional security, robustness, capacity, and flexibility. The framework is available at https://xuanyuzhang21.github.io/project/gshider/. GS-Hider is the first method to study 3DGS steganography, offering applications in encrypted transmission, 3D compression, and copyright protection. The framework can hide an image, one or multiple 3D scenes in a single 3D scene, and is secure against steganography detection. It is also robust to degradation and can hide multiple scenes in a single scene without interference. The framework is designed to be flexible and efficient, with high rendering speed and fidelity.