The spindle assembly checkpoint (SAC) delays anaphase until all chromosomes are bioriented on the mitotic spindle. Under current models, unattached kinetochores transduce the SAC by catalyzing the intramitotic production of a diffusible inhibitor of APC/C^Cdc20 (the anaphase-promoting complex/cyclosome and its coactivator Cdc20, a large ubiquitin ligase). Here we show that nuclear pore complexes (NPCs) in interphase cells also function as scaffolds for anaphase-inhibitory signaling. This role is mediated by Mad1-Mad2 complexes tethered to the nuclear basket, which activate soluble Mad2 as a binding partner and inhibitor of Cdc20 in the cytoplasm. Displacing Mad1-Mad2 from nuclear pores accelerated anaphase onset, prevented effective correction of merotelic errors, and increased the threshold of kinetochore-dependent signaling needed to halt mitosis in response to spindle poisons. A heterologous Mad1-NPC tether restored Cdc20 inhibitor production and normal M phase control. We conclude that nuclear pores and kinetochores both emit "wait anaphase" signals that preserve genome integrity.