The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La${}_3$Ni${}_2$O${}_7$ has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) phase of La${}_3$Ni${}_2$O${}_7$. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic $Cmmm$ to the tetragonal $P4/mmm$ space group at 13~GPa. Above 19 GPa, the phase shows a clear superconducting transition, confirmed by a zero-resistance state, albeit at a much lower temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results not only expand the family of RP nickelate superconductors but also deepen the understanding of the superconducting mechanism that governs the transition temperature in these materials.