Perovskite/silicon tandem solar cells (PSTSCs) are among the most promising photovoltaic technologies for surpassing the efficiency limits of single-junction solar cells. A major breakthrough in this field has been the introduction of ultrathin hole-selective self-assembled monolayers (HS-SAMs). These molecular interfacial layers enable precise energy-level alignment, efficient hole extraction, and effective passivation of interfacial defects, while remaining compatible with textured silicon and large-area substrates. In this review, we discuss the molecular design principles of HS-SAMs, including anchoring groups, conjugated donor units, linkers, dipole engineering, wettability, and packing density, highlighting how these parameters influence charge extraction, perovskite crystallization, interfacial passivation, and device stability. We also summarize key advances in PSTSCs enabled by HS-SAMs, including a certified PCE of 29.2% with Me-4PACz and efficiencies above 32% with optimized PACz derivatives and 32.5% with 2PACz/IZO interlayers, together with operational stability exceeding 1000 h under damp-heat or continuous illumination conditions. By consolidating recent progress in molecular engineering, interfacial passivation, scalable deposition, and manufacturability, this review identifies HS-SAMs as a cornerstone for the development of efficient, stable, and commercially relevant perovskite and perovskite/silicon tandem photovoltaics.