KISS FFT is a mixed-radix Fast Fourier Transform library implemented in C and C++. It provides a compact, portable, and reasonably efficient FFT implementation supporting complex and real-valued transforms in one or multiple dimensions. The library is designed to be simple to integrate, offering both a C API (kiss_fft, kiss_fftr, kiss_fftnd, kiss_fftndr) and a header-only C++ template class. It supports multiple scalar types (float, double, int16_t, int32_t) and SIMD acceleration via SSE, LSX, and LASX instruction sets. The core algorithm uses decimation-in-time with optimized butterflies for radices 2, 3, 4, and 5, and provides a cache mechanism (kfc) to reuse FFT configurations. The codebase is self-contained with no external dependencies beyond libm; optional tools require fftw3 and libpng for benchmarking and image generation. It is widely used in embedded systems, audio processing, and scientific computing due to its permissive BSD license and straightforward API.