Substrate rigidity modulates cell mechanics, which affect cell migration and proliferation. Quantifying the effects of substrate rigidity on cancer cell mechanics requires a quantifiable parameter that can be measured for individual cells, as well as a substrate platform with rigidity being the only variable. Here we used single-cell force spectroscopy to pull cancer cells on substrates varying only in rigidity, and extracted a parameter from the force-distance curves to be used to quantify the properties of membrane tethers. Our results showed that tether force increases with substrate rigidity until it reaches its asymptotic limit. The variations are similar for all three cancer cell lines studied, and the largest change occurs in the rigidity regions of softer tissues, indicating a universal response of cancer cell elasticity to substrate rigidity.