N⁶-methyladenosine (m⁶A) is the most frequent of the 160 RNA modifications reported so far. Accumulating evidence suggests that the METTL3/METTL14 protein complex, part of the m⁶A regulation machinery, is a key player in a variety of diseases including several types of cancer, type 2 diabetes, and viral infections. Here we report on a protein crystallography-based medicinal chemistry optimization of a METTL3 hit compound that has resulted in a 1400-fold potency improvement (IC₅₀ of 5 nM for the lead compound 22 (UZH2) in a time-resolved Förster resonance energy transfer (TR-FRET) assay). The series has favorable ADME properties as physicochemical characteristics were taken into account during hit optimization. UZH2 shows target engagement in cells and is able to reduce the m⁶A/A level of polyadenylated RNA in MOLM-13 (acute myeloid leukemia) and PC-3 (prostate cancer) cell lines.