  The charge frustration with the mixed-valence state inherent to
LiTi$_2$O$_4$, which is found to be a unique spinel oxide superconductor, is
the impetus for paying special attention to reveal the existence of intriguing
superconducting properties. Here, we report a pronounced fourfold rotational
symmetry of the superconductivity in high-quality single-crystalline
LiTi$_2$O$_4$ (001) thin films. Both the magnetoresistivity and upper critical
field under an applied magnetic field manifest striking fourfold oscillations
deep inside the superconducting state, whereas the anisotropy vanishes in the
normal state, demonstrating that it is an intrinsic property of the
superconducting phase. We attribute this behavior to the unconventional
$d$-wave superconducting Cooper pairs with the irreducible representation of
$E_g$ protected by $O_h$ point group in LiTi$_2$O$_4$. Our findings demonstrate
the unconventional character of the pairing interaction in a three-dimensional
spinel oxide superconductor and shed new light on the pairing mechanism of
unconventional superconductivity.
