Hypoxia is a feature of most solid tumors and a key determinant of cancer growth and propagation. Sensing hypoxia effectively could lead to more favorable clinical outcomes. Here, we report a molecular antenna-based bimodal probe designed to exploit the complementary advantages of magnetic resonance (MR)- and optical-based imaging. Specifically, we describe the synthesis and evaluation of a dual-action probe (NO2-Eu) that permits hypoxia-activated chemical exchange saturation transfer (CEST) MR and optical imaging. In CT26 cells, this NO2-Eu probe not only provides an enhanced CEST MRI signal but also turns "on" the optical signal under hypoxic conditions. Time-dependent in vivo CEST imaging in a hypoxic CT26 tumor xenograft mouse model revealed probe-dependent tumor detection by CEST MRI contrast in the tumor area. We thus suggest that dual-action hypoxia probes, like that reported here, could have a role to play in solid tumor diagnosis and monitoring.