Mechanical forces influence many aspects of cell behavior. Forces are detected and transduced into biochemical signals by force bearing molecular elements located at the cell surface, in adhesion complexes or in cytoskeletal structures1. The nucleus is physically connected to the cell surface through the cytoskeleton and the linker of nucleoskeleton and cytoskeleton (LINC) complex, allowing rapid mechanical stress transmission from adhesions to the nucleus2. Whereas it has been demonstrated that nuclei experience force3, the direct effect of force on the nucleus is not known. Here we show that isolated nuclei are able to respond to force by adjusting their stiffness to resist the applied tension. Using magnetic tweezers, we found that applying force on nesprin-1 triggers nuclear stiffening that does not involve chromatin or nuclear actin, but requires an intact nuclear lamina and emerin, a protein of the inner nuclear membrane. Emerin becomes tyrosine phosphorylated in response to force and mediates the nuclear mechanical response to tension. Our results demonstrate that mechanotransduction is not restricted to cell surface receptors and adhesions but can occur within the nucleus.