Background: NKG2D is an activating receptor expressed on all human natural killer (NK) cells and CD8 T-cells. It is widely accepted that human tumors shed soluble NKG2D ligands, such as soluble major histocompatibility complex-I chain-related molecule (sMIC), to evade immune activation. Harnessing the NKG2D/NKG2D-ligand axis has emerged as a promising avenue for immunotherapy. However, controversy exists over whether soluble NKG2D ligands are immunosuppressive or immunostimulatory. Methods: Cell lines for melanoma (B16F10), Lewis lung carcinoma (LLC1), and prostate cancer (TRAMP-C2, T-C2) were used as parental cell lines in this study. Derivative cell lines over-expressing the soluble human NKG2D ligand sMICB (B16F10-sMICB, TC2-sMICB and LLC-sMICB) were created. We inoculated these 3 syngeneic transplantable tumor models and their respective parental lines into mice and compared tumor incidence/growth over time. We also compared surface markers representative of cell stemness (SCA-1, CD166 and CD133) in mouse cancer cells between the sMICB-expressing tumor cells and their respective parental cell lines. We then characterized the tumor immune microenvironment by flow cytometry. Results: B16F10-sMICB mice demonstrated a significant delay in tumor development compared to control mice (23 vs. 6 days, p < 0.05). Over longer follow-up, rapid tumorigenesis still occurred in the experimental group, with a 50% tumor incidence by day 26. By contrast, the TRAMP-C2-sMICB mice demonstrated higher tumorigenic ability (p < 0.05). LLC-sMICB mice demonstrated no difference in tumorigenicity. Time to tumor onset was more strongly associated with expression of stemness-like markers rather than sMIC. Once tumors were established, they grew significantly more aggressively in the TRAMP-C2-sMICB, LLC-sMICB and B16F10-sMICB lines vs. their parental counterparts. sMICB expressing tumors contained significantly lower percentages of NK cells and CD8 T-cells in tumor infiltrates than their respective parental lines. NK cells and CD8 T Cells from sMICB-expressing tumors had an impaired capacity to produce interferon gamma. Conclusions: Expression of a soluble NKG2D ligand had minimal impact on tumor establishment but facilitated more aggressive growth of established tumors due to immune suppression. Soluble NKG2D ligands imprint negative anti-tumor immunity onto established tumors. Our data differentiated the impact of NKG2D ligands on tumor onset vs. tumor progression and reinforced the translational concept that soluble NKG2D ligands are strong therapeutic targets for future study.