Deafness is generally associated with poor mental arithmetic, possibly due to neuronal differences in arithmetic processing across language modalities. Here, we investigated for the first time the neuronal networks supporting arithmetic processing in adult deaf signers. Deaf signing adults and hearing non-signing peers performed arithmetic and phonological tasks during fMRI scanning. At whole brain level, activation patterns were similar across groups. Region of interest analyses showed that although both groups activated phonological processing regions in the left inferior frontal gyrus to a similar extent during both phonological and multiplication tasks, deaf signers showed significantly more activation in the right horizontal portion of the inferior parietal sulcus. This region is associated with magnitude manipulation along the mental number line. This pattern of results suggests that deaf signers rely more on magnitude manipulation than hearing non-signers during multiplication, but that phonological involvement does not differ significantly between groups.Abbreviations: AAL: Automated Anatomy Labelling; fMRI: functional magnetic resonance imaging; HIPS: horizontal portion of the intraparietal sulcus; lAG: left angular gyrus; lIFG: left inferior frontal gyrus; rHIPS: right horizontal portion of the intraparietal sulcus