Since the detection of unexpectedly high deuterium fractionation in the Galactic Center almost 20 years ago, many follow up observations were carried out in different Galactic environments which showed deuterium abundances well above the elemental ratio D/H~1.5x10^-5. These observations have been well reproduced by chemical models and showed that deuteration is an excellent tracer of the degree of chemical processing of the gas by star formation, and therefore a tracer of pristine material feeding the future star formation events. Though models have been proposed for different extragalactic environments, only now, with the superb sensitivity of ALMA, observations are possible. We report for the first time detection of deuterated molecules in a external galaxy, NGC 253, and present the derived D/H ratio. This data have allowed us to confront chemical models and observations in uncharted regions of the parameters space, and to constrain chemical evolution models of star-forming regions. Given the key role of deuterated molecules, our study makes an important contribution in understanding the cosmic chemical enrichment.