FIP200 (focal adhesion kinase [FAK] family interacting protein of 200 kD) is a newly identified protein that binds to the kinase domain of FAK and inhibits its kinase activity and associated cellular functions. Here, we identify an interaction between FIP200 and the TSC1-TSC2 complex through FIP200 binding to TSC1. We found that association of FIP200 with the TSC1-TSC2 complex correlated with its ability to increase cell size and up-regulate S6 kinase phosphorylation but was not involved in the regulation of cell cycle progression. Conversely, knockdown of endogenous FIP200 by RNA interference reduced S6 kinase phosphorylation and cell size, which required TSC1 but was independent of FAK. Furthermore, overexpression of FIP200 reduced TSC1-TSC2 complex formation, although knockdown of endogenous FIP200 by RNA interference did not affect TSC1-TSC2 complex formation. Lastly, we showed that FIP200 is important in nutrient stimulation-induced, but not energy- or serum-induced, S6 kinase activation. Together, these results suggest a cellular function of FIP200 in the regulation of cell size by interaction with the TSC1-TSC2 complex.