The neurofibromatosis type 1 (NF1) tumor suppressor (neurofibromin) is thought to play crucial roles in cellular Ras‐ and cAMP‐dependent kinase (PKA)‐associated signals. In this study, we identified a cellular neurofibromin‐associating protein, N ^G,N ^G‐dimethylarginine dimethylaminohydrolase (DDAH) that is known as a cellular NO/NOS regulator. The interaction of DDAH was mainly directed to the C‐terminal domain (CTD) and to the cysteine/serine‐rich domain (CSRD) of neurofibromin, coinciding with the regions containing specific PKA phosphorylation sites. DDAH increased PKA phosphorylation of native neurofibromin in a dose‐dependent manner, especially affecting the phosphorylation of CSRD. These findings suggest that the PKA accessibility of neurofibromin was regulated via DDAH interaction, and this regulation may modulate the cellular function of neurofibromin that is implicated in NF1‐related pathogenesis.