The objective of the present study was to explore the molecular mechanism with which a single nucleotide polymorphism (rs61764370) interferes with the interaction between the 3'-untranslated region (3'-UTR) of Kirsten rat sarcoma viral oncogene homolog (KRAS) and let-7a, and its association with the metastasis of osteosarcoma (OS). In this study, we confirmed that KRAS is a target of let-7a in OS cells, and the introduction of rs61764370 minor allele into KRAS 3'-UTR significantly compromised the microRNA (miRNA)/mRNA interaction using a luciferase reporter system. Additionally, a total of 36 OS tissue samples of three different genotypes (TT,22; TG,10; GG,4) were obtained, and the expression of let-7a and KRAS was determined. We showed that let-7a mRNA expression was similar between each group whereas the mRNA and protein expression of KRAS in the TT genotype group was significantly lower than that in the GT or GG genotype groups. Moreover, we identified a negative regulatory relationship between let-7a and KRAS. Furthermore, we demonstrated that let-7a and KRAS interfered with the viability, invasiveness and migration of OS cells genotyped as TT. In the OS cells genotyped as TG, let-7a exerted minimal effects, and the effect of KRAS siRNA remained. Taken together, the findings of the present study demonstrated that the KRAS 3'-UTR rs61764370 polymorphism interfered with miRNA/mRNA interaction, and showed that the minor allele was associated with an elevated risk of developing metastatic disease in OS.