Context : The presence of a small-mass planet ( M _ { p } < 0.1 M _ { Jup } ) seems , to date , not to depend on metallicity , however , theoretical simulations have shown that stars with subsolar metallicities may be favoured for harbouring smaller planets . A large , dedicated survey of metal-poor stars with the HARPS spectrograph has thus been carried out to search for Neptunes and super-Earths . Aims : In this paper , we present the analysis of HD175607 , an old G6 star with metallicity [ Fe/H ] = -0.62 . We gathered 119 radial velocity measurements in 110 nights over a time span of more than nine years . Methods : The radial velocities were analysed using Lomb-Scargle periodograms , a genetic algorithm , a Markov chain Monte Carlo analysis , and a Gaussian processes analysis . The spectra were also used to derive stellar properties . Several activity indicators were analysed to study the effect of stellar activity on the radial velocities . Results : We find evidence for the presence of a small Neptune-mass planet ( M _ { p } \sin i = 8.98 \pm 1.10 M _ { \oplus } ) orbiting this star with an orbital period P = 29.01 \pm 0.02 days in a slightly eccentric orbit ( e = 0.11 \pm 0.08 ) . The period of this Neptune is close to the estimated rotational period of the star . However , from a detailed analysis of the radial velocities together with the stellar activity , we conclude that the best explanation of the signal is indeed the presence of a planetary companion rather than stellar related . An additional longer period signal ( P \sim 1400 d ) is present in the data , for which more measurements are needed to constrain its nature and its properties . Conclusions : HD 175607 is the most metal-poor FGK dwarf with a detected low-mass planet amongst the currently known planet hosts . This discovery may thus have important consequences for planet formation and evolution theories .