Abstract BRAF mutations play a well-established role in melanomagenesis; however, without additional genetic alterations, tumor development is restricted by oncogene-induced senescence (OIS). Here, we show that mutations in the NF1 tumor suppressor gene cooperate with BRAF mutations in melanomagenesis by preventing OIS. In a genetically engineered mouse model, Nf1 mutations suppress Braf-induced senescence, promote melanocyte hyperproliferation, and enhance melanoma development. Nf1 mutations function by deregulating both phosphoinositide 3-kinase and extracellular signal–regulated kinase pathways. As such, Nf1/Braf–mutant tumors are resistant to BRAF inhibitors but are sensitive to combined inhibition of mitogen-activated protein/extracellular signal–regulated kinase kinase and mTOR. Importantly, NF1 is mutated or suppressed in human melanomas that harbor concurrent BRAF mutations, NF1 ablation decreases the sensitivity of melanoma cell lines to BRAF inhibitors, and NF1 is lost in tumors from patients following treatment with these agents. Collectively, these studies provide mechanistic insight into how NF1 cooperates with BRAF mutations in melanoma and show that NF1/neurofibromin inactivation may have an impact on responses to targeted therapies. Significance: This study elucidates the mechanism by which NF1 mutations cooperate with different BRAF mutations in melanomagenesis and shows that NF1/neurofibromin loss may desensitize tumors to BRAF inhibitors. Cancer Discov; 3(3); 338–49. ©2012 AACR. See related commentary by Gibney and Smalley, p. 260 This article is highlighted in the In This Issue feature, p. 239