  Bell's theorem states that no local hidden variable model is compatible with
quantum mechanics. Surprisingly, even if we release the locality constraint,
certain nonlocal hidden variable models, such as the one proposed by Leggett,
may still be at variance with the predictions of quantum physics. Here, we
report an experimental test of Leggett's nonlocal model with solid-state spins
in a diamond nitrogen-vacancy center. We entangle an electron spin with a
surrounding weakly coupled $^{13}C$ nuclear spin and observe that the entangled
states violate Leggett-type inequalities by more than four and seven standard
deviations for six and eight measurement settings, respectively. Our
experimental results are in full agreement with quantum predictions and violate
Leggett's nonlocal hidden variable inequality with a high level of confidence.
