  The recent observation of superconductivity in thin film infinite-layer
nickelates$^{1-3}$ offers a different angle to investigate superconductivity in
layered oxides$^{4}$. A wide range of candidate models have been
proposed$^{5-10}$, emphasizing single- or multi-orbital electronic structure,
Kondo or Hund's coupling, and analogies to cuprates. Clearly, further
experimental characterization of the superconducting state is needed to develop
a full understanding of the nickelates. Here we use magnetotransport
measurements to probe the superconducting anisotropy in
Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$. We find that the upper critical field is
surprisingly isotropic at low temperatures despite the layered crystal
structure. In a magnetic field the superconductivity is strongly Pauli-limited,
such that the paramagnetic effect dominates over orbital de-pairing. Underlying
this isotropic response is a substantial anisotropy in the superconducting
coherence length, which is at least four times longer in-plane than
out-of-plane. A prominent low-temperature upturn in the upper critical field
indicates the presence of an unconventional ground state.
