With its relatively small key size, elliptic-curve cryptography (ECC) is considered as the public-key cryptographic algorithm of choice for wireless sensor networks (WSNs). In this work, we implemented ECC in the frequency domain, that is,by using the number theoretic transform, and without using hardware multiplier support, on the constrained MSP430 microcontroller widely used in WSNs. Our 169-bit ECC implementation uses Edwards curves and performs scalar point multiplication in only 1.97 and 0.98s for multiplication of random and fixed points, respectively. Unlike many implementations in literature, our implementation does not use hardware multiplier support, which makes it desirable for low-power applications on constrained WSN platforms. To our knowledge, this study presents the first ever software implementation of ECC in the frequency domain on a constrained low-power microcontroller without hardware multiplier support. Copyright (C) 2016 John Wiley & Sons, Ltd.