AbstractPerovskite nickelatesRNiO3(R= rare-earth ion) exhibit complex rare-earth ion dependent phase diagram and high tunability of various appealing properties. Here, combining first- and finite-temperature second-principles calculations, we explicitly demonstrate that the superior merits of the interplay among lattice, electron, and spin degrees of freedom can be passed toRNiO2, which recently gained significant interest as superconductors. We unveil that decreasing the rare-earth size directly modulates the structural, electronic, and magnetic properties and naturally groups infinite-layer nickelates into two categories in terms of the Fermi surface and magnetic dimensionality: compounds with large rare-earth sizes (La, Pr) closely resemble the key properties of CaCuO2, showing quasi-two-dimensional (2D) antiferromagnetic (AFM) correlations and strongly localizeddx2−y2orbitals around the Fermi level; the compounds with small rare-earth sizes (Nd–Lu) are highly analogous to ferropnictides, showing three-dimensional (3D) magnetic dimensionality and strongkzdispersion ofd3z2−r2electrons at the Fermi level. Additionally, we highlight thatRNiO2withR= Nd–Lu exhibit on cooling a structural transition with the appearance of oxygen rotation motion, which is softened by the reduction of rare-earth size and enhanced by spin-rotation couplings. The rare-earth control ofkzdispersion and structural phase transition might be the key factors differentiating the distinct upper critical field and resistivity in different compounds. The established original phase diagram summarizing the temperature and rare-earth controlled structural, electronic, and magnetic transitions inRNiO2compounds provides rich structural and chemical flexibility to tailor the superconducting property.