Scanning tunneling microscopy (STM) gives atomic resolution for physical properties such as the electronic density of states, spin polarization, and spin and vibrational excitations. We aim to extend these capabilities to include magnetic field sensing by using electron spin resonance (ESR) of a tip-attached molecular spin. ESR in an STM has been used previously to sense spin resonance in individual transition metal atoms and small molecules deposited on the surface, making each spin center a sensitive local magnetic field detector of its atomic-scale neighborhood. Sufficient spin coherence times are obtained by using a thin epitaxial insulating film to controllably isolate the resonant spin centers from the conducting substrate. We explore the use of molecular spins transferred intact to the STM tip. The molecule's structure provides the required isolation from the metallic tip's conduction electrons. Scanning ESR should serve as versatile tool for investigating atomic-scale magnetism by detecting the local magnetic fields of molecules, self-assembled layers, and custom-assembled nanostructures. *We acknowledge financial support from the Office of Naval Research.