Synthesis of silver nanowires (Ag NWs) has been studied for decades. However, Ag NWs with diameters below 20 nm synthesised by simple and robust approaches are still rarely reported. In this work, Ag NWs with an average diameter of ∼15 nm and an aspect ratio of over 1000 have been prepared by using a Grignard reagent 5-chloro-2-thienylmagnesium bromide as an assistant additive. In particular, the presence of 5-chloro-2-thienylmagnesium bromide is proven to be beneficial for the in situ formation of smaller AgBr and AgCl particles step by step in comparison with traditional inorganic halide ions, and then promote the final growth of ultrafine Ag NWs.