Alternative splicing of the pyruvate kinase M gene involves a choice between mutually exclusive exons 9 and 10. Use of exon 10 to generate the M2 isoform is crucial for aerobic glycolysis (the Warburg effect) and tumour growth. We previously demonstrated that splicing enhancer elements that activate exon 10 are mainly found in exon 10 itself, and deleting or mutating these elements increases the inclusion of exon 9 in cancer cells. To systematically search for new enhancer elements in exon 10 and develop an effective pharmacological method to force a switch from PK-M2 to PK-M1, we carried out an antisense oligonucleotide (ASO) screen. We found potent ASOs that target a novel enhancer in exon 10 and strongly switch the splicing of endogenous PK-M transcripts to include exon 9. We further show that the ASO-mediated switch in alternative splicing leads to apoptosis in glioblastoma cell lines, and this is caused by the downregulation of PK-M2, and not by the upregulation of PK-M1. These data highlight the potential of ASO-mediated inhibition of PK-M2 splicing as therapy for cancer.