Diffuse intrinsic pontine glioma (DIPG) is an incurable pediatric brain tumor, with approximately 25% of DIPGs harboring activating ACVR1 mutations that commonly co-associate with H3.1K27M mutations. Here we show that in vitro expression of ACVR1 R206H with and without H3.1K27M upregulates mesenchymal markers and activates Stat3 signaling. In vivo expression of ACVR1 R206H or G328V with H3.1K27M and p53 deletion induces glioma-like lesions but is not sufficient for full gliomagenesis. However, in combination with PDGFA signaling, ACVR1 R206H and H3.1K27M significantly decrease survival and increase tumor incidence. Treatment of ACVR1 R206H mutant DIPGs with exogenous Noggin or the ACVR1 inhibitor LDN212854 significantly prolongs survival, with human ACVR1 mutant DIPG cell lines also being sensitive to LDN212854 treatment. Together, our results demonstrate that ACVR1 R206H and H3.1K27M promote tumor initiation, accelerate gliomagenesis, promote a mesenchymal profile partly due to Stat3 activation, and identify LDN212854 as a promising compound to treat DIPG.