The molecular regulation of growth of non-small cell lung cancer (NSCLC) has not been fully clarified. In NSCLC, we detected significantly higher levels of activating protein-4 (AP4), significantly lower levels of p21, and significantly lower levels of phosphorylated SMAD2 as an indicator of activated transforming growth factor β (TGFβ) receptor signaling, compared to the adjacent normal lung tissue. Moreover, a strong negative correlation was detected between AP4 and p21 levels. Since p21 is a potent cell-cycle inhibitor, we were thus promoted to examine the relationships among AP4, TGFβ receptor signaling, and cell growth in NSCLC. Using a human NSCLC cell line HepG2 cells, we found that activation of TGFβ receptor signaling increased p21 levels through phosphorylation of SMAD2. Moreover, AP4 inhibited phosphorylation of SMAD2 to contradict the effect of activated TGFβ receptor signaling on cell growth inhibition in NSCLC. Furthermore, binding of TGFβ1 to its receptor also directly increased AP4 transcription, which appeared to negatively control the levels of activated TGFβ receptor signaling. Taken together, our results suggest that AP4 may inhibit the phosphorylation of SMAD2, which is induced by receptor binding with TGFβ1, to abolish the inhibitory effect of activated TGFβ receptor signaling on cell growth in NSCLC. Our study thus highlights AP4 as a novel therapeutic target for NSCLC.