This study was focused on investigating the anticancer potential of indole-3-carbinol (I3C) against osteosarcoma MG-63 and U2OS cells. A wound healing assay indicated that IC3 inhibited migration of MG-63 and U2OS cells. MTT, WST-1, and colony formation assays revealed that treatment of MG-63 and U2OS cells with I3C decreased cell viability. Fluorescence-activated cell sorting (FACS) analysis showed that I3C induced apoptosis in a dose- and time-dependent manner in MG-63 and U2OS cells. Moreover, via terminal deoxynucleotidyl transferase- (TdT-) mediated dUTP-biotin nick-end labeling (TUNEL) assay, we detected that I3C induced DNA fragmentation. Western blotting demonstrated that activated forms of caspase-3, caspase-7, and caspase-9, as well as poly (ADP-ribose) polymerase (PARP) were increased in MG-63 and U2OS cells, following treatment with I3C. Furthermore, protein expression levels of FOXO3, Bax, and Bim extra-large form were increased while those of Akt, JNK, p38, phosphorylated ERK, and Bcl-xL were decreased by I3C treatment in MG-63 and U2OS cells. Thus, the study indicates that I3C may induce apoptosis in human osteosarcoma MG-63 and U2OS cells via the activation of apoptotic signaling pathways by FOXO3.