  The interplay between dimensionality and various phases of matter is a
central inquiry in condensed matter physics. New phases are often discovered
through spontaneously broken symmetry. Understanding the dimensionality of
superconductivity in the high-temperature cuprate analogue $-$ layered
nickelates and revealing a new symmetry-breaking state are the keys to
deciphering the underlying pairing mechanism. Here, we demonstrate the
highly-tunable dimensionality and a broken rotational symmetry state in the
superconductivity of square-planar layered nickelates. The superconducting
state, probed by superconducting critical current and magnetoresistance within
superconducting transition under direction-dependent in-plane magnetic fields,
exhibits a $C_2$ rotational symmetry which breaks the $C_4$ rotational symmetry
of the square-planar lattice. Furthermore, by performing detailed examination
of the angular dependent upper critical fields at temperatures down to 0.5 K
with high-magnetic pulsed-fields, we observe a crossover from two-dimensional
to three-dimensional superconducting states which can be manipulated by the
ionic size fluctuations in the rare-earth spacer layer. Such a large degree of
controllability is desired for tailoring strongly two/three-dimensional
superconductors and navigating various pairing landscapes for a better
understanding of the correlation between reduced dimensionality and
unconventional pairing. These results illuminate new directions to unravel the
high-temperature superconducting pairing mechanism.
