PopSift is a CUDA-accelerated SIFT (Scale-Invariant Feature Transform) feature extraction library. It provides a C++ library (libpopsift) with an asynchronous API for enqueuing images and retrieving features, and two command-line demo applications (popsift-demo, popsift-match). The library implements the full SIFT pipeline (Gaussian pyramid, extrema detection, orientation assignment, descriptor computation) on NVIDIA GPUs, supporting multiple algorithmic variants (PopSift, VLFeat, OpenCV) for research comparison. It is designed for high-performance real-time feature extraction and can be integrated via CMake. Despite its research origins (Simula Research Laboratory), the codebase is well-structured with modular CUDA kernels, debug utilities, and parallel primitives. The library is actively used in computer vision projects and maintains compatibility with recent CUDA versions while preserving legacy fallbacks.