Modulation of protein-RNA interaction (PRI) using small molecules is a promising strategy to develop therapeutics. LIN28 is an RNA-binding protein that blocks the maturation of the tumor suppressor let-7 microRNAs. Herein, we performed a fluorescence polarization-based screening and identified trisubstituted pyrrolinones as small-molecule inhibitors disrupting the LIN28-let-7 interaction. The most potent compound C902 showed dose-dependent inhibition in an EMSA validation assay, enhanced thermal stability of the cold shock domain of LIN28, and increased mature let-7 levels in JAR cells. The structure-activity relationship study revealed key structural features contributing to either PRI inhibition or stabilization of protein-protein interaction (PPI). The pyrrolinones identified in this study not only represent a new class of LIN28-binding molecules that diversify the limited available LIN28 inhibitors but also represent the first examples of small molecules that showed substituent-dependent PRI inhibitory and PPI activating activities.