This study was conducted to determine the effects of seed maturity stages depending on different harvest dates on fatty acid compositions of fenugreek (Trigonella foenum-graecum L.) and black cumin (Nigella sativa L.) with the aid of chemometric methods. Principal component analysis (PCA) and Agglomerative Hierarchical Clustering (AHC) analysis were used to assess and compare fatty acid compositions of fenugreek and black cumin seeds. Experiments for fenugreek and black cumin plants were separately conducted side-by side. For both plant species, 9 different harvest dates at 3 seed maturity stages (immature, premature, and full mature) were experimented. Following the harvests, plant height, single plant weight, crude oil content and fatty acid compositions were determined. Plant heights varied between 45.10-63.37 cm for fenugreek and between 34.57-43.67 cm for black cumin; crude oil contents varied between 4.01-4.95% for fenugreek and between 18.42-40.35% black cumin. Present findings revealed that immature stage had positive effects on crude oil content of fenugreek seeds and premature stage on crude oil contents of black cumin seeds. Also, 9 fatty acid components were identified in fenugreek and 7 in black cumin seeds. In both plant species, linoleic acid (C18:2) was the major component. With the greatest UFA (total unsaturated fatty acids) and the lowest SFA (total saturated fatty acids) ratios, F8 (19 July 2019) harvest date could be recommended for fenugreek plants and B4 (17 July 2019) for black cumin plants. PCA and AHC analysis revealed that with regard to fatty acid composition, F3 (08 July 2019) harvest date was different from the others in fenugreek and B1 (10 July 2019) in black cumin plants. It was concluded based on present findings that seed maturity stages had significant effects on fatty acid composition of fenugreek and black cumin seeds.