Measurement of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>g</mml:mi> </mml:math> Factor of Ground-State <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mmultiscrip

We demonstrate nuclear magnetic resonance of optically trapped ground-state ultracold ^{87}Sr atoms. Using a scheme in which a cloud of ultracold ^{87}Rb is co-trapped nearby, we improve the determination of the nuclear g factor, g_{I}, of atomic ^{87}Sr by more than two orders of magnitude, reaching accuracy at the parts-per-million level. We achieve similar accuracy in the ratio of relevant g factors between Rb and Sr. This establishes ultracold ^{87}Sr as an excellent linear in-vacuum magnetometer. More generally, our Letter demonstrates how ultracold neutral atoms can be used for the precise determination of highly relevant atomic and nuclear magnetic properties. These results are important for ongoing efforts toward quantum simulation, quantum computation and optical atomic clocks employing ^{87}Sr, and other neutral alkaline-earth and alkaline-earth-like atoms.