We present a simple solution processed synthesis route for GaAs nanocrystals (NCs) with narrow size distribution and high crystallinity using wet chemical methods and commercially available inexpensive precursors with reduced toxicity. The reaction pathway can be described in three steps, starting with a transmetalation reaction between the gallium(III) halide precursor GaCl3 and the reduction agent n-butyllithium. At elevated temperatures elemental gallium is released in this process and enables the formation of GaAs NCs with magnesium arsenide (Mg3As2) as the arsenic source. We obtained a variety of different III-V semiconductor NCs including GaAs, InP, In As, and GaP using this transmetalation reaction pathway.