Transcription by RNA polymerase II (RNA Pol II) is a tightly regulated process in eukaryotes. A recent study revealed that the trimethylation of histone H3 at lysine 4 (H3K4me3) activates gene expression by promoting RNA Pol II pause-release and transcription elongation rather than transcription initiation. However, the molecular mechanisms linking H3K4me3 to transcription elongation remain largely unclear. Here, we show that an H3K4me3 reader regulates this process. A genetic screening in Arabidopsis reveals that root growth in salicylic acid 1 (RGSA1) is required for the transcription of genes induced by the plant hormone salicylic acid. RGSA1 contains a cysteine-tryptophan domain, which is essential for its binding to H3K4me3. RGSA1 mainly occupies transcription start sites, where it recruits the positive transcription elongation factor B (P-TEFb) complex to facilitate transcription elongation. Given the evolutionary conservation of H3K4me3 readers and P-TEFb, our findings may represent a conserved mechanism underlying transcription elongation across eukaryotes.