We present libNMF -- a computationally efficient high performance library for computing nonnegative matrix factorizations (NMF) written in C. Various algorithms and algorithmic variants for computing NMF are supported. libNMF is based on external routines from BLAS (Basic Linear Algebra Subprograms), LAPack (Linear Algebra package) and ARPack, which provide efficient building blocks for performing central vector and matrix operations. Since modern BLAS implementations support multi-threading, libNMF can exploit the potential of multi-core architectures. In this paper, the basic NMF algorithms contained in libNMF and existing implementations found in the literature are briefly reviewed. Then, libNMF is evaluated in terms of computational efficiency and numerical accuracy and compared with the best existing codes available. libNMF is publicly available at http://rlcta.univie.ac.at/software.