Liquid–liquid phase separation (LLPS) of intrinsically disordered proteins (IDPs) and the action of molecular chaperones are tightly connected. An important class of molecular chaperones are peptidyl prolyl isomerases, which enhance the cis/trans-isomerization of proline. However, little is known about the impact of peptidyl prolyl isomerases on the LLPS of IDPs, which often contain many prolines. Here, we demonstrate that the most ubiquitous peptidyl prolyl isomerase, peptidyl prolyl isomerase A (PPIA), concentrates inside liquid-like droplets formed by the Alzheimer’s disease-associated protein tau, as well as inside RNA-induced coacervates of a proline–arginine dipeptide repeat protein. We further show that the recruitment of PPIA into the IDP droplets triggers their dissolution and return to a single mixed phase. NMR-based binding and proline isomerization studies provide insights into the mechanism of LLPS modulation. Together, the results establish a regulatory role of proline isomerases on the liquid–liquid phase separation of proline-rich IDPs.