THIS INVESTIGATION deals with assessment of the diversity of 14 populations representing eleven species of Solanum from southwest Saudi Arabia based on differences in the secondary metabolites by using GC-MS analysis. The analysis was carried out using ethanol extract of the examined Solanum species/populations and 87 different phyto-constituents were detected at six different retention times. The highest M.wt. for the identified compounds was 641 and was recorded in S. villosum at a retention time of 15 min; its formula is C38H35N5O5 and its decided name is N-benzoyl-9-(2,3,5-tri-O-benzyl pentofuranosyl)-9H-purin-6-amine. On the other hand, the lowest M.wt. for the identified compounds was 84; its formula is C6H12 and its decided name is 1Hexane and it was recorded in all Solanum species/populations except the two populations of S. incanum, S. coagulans and S. schimperianum at the retention time of 5 min. Based on differences in the phyto-constituents, genetic similarity coefficients were calculated and two distance trees were constructed to illustrate the relatedness of the examined species. The results support a hypothesis that S. villosum and S. nigrum can be regarded as one complex species. The results also revealed that S. coagulans is related to S. macracanthum and S. glabratum and also S. schimperianum is related to S. incanum. The results also revealed that S. torvum, S. sisymbriifolium and S. dulcamara are closely related species. This is generally congruent with the relatedness of the examined specie based on morphological variation and to some extend agree with their systematic treatments.