Higher serum phosphorous is associated with cerebral small vessel disease, an important driver of cognitive decline and dementia. Whether serum phosphorous, a potentially modifiable parameter, associates with risk of incident dementia is not known. We aimed to examine the association between serum phosphorous and risk of incident dementia and to determine if the association is modified by age. We used the United States Department of Veterans Affairs national databases to build a longitudinal observational cohort of US veterans without prior history of dementia and with at least one outpatient serum phosphorus between October 2008 and September 2010 and followed them until September 2014. Serum phosphorus was categorized into quintiles: 2.9, >2.9 to 3.2, >3.2 to 3.5, >3.5 to 3.9, >3.9 mg/dL. There were 744,235 participants in the overall cohort. Over a median follow- up of 5.07 years (Interquartile range [IQR]: 4.28, 5.63), adjusted Cox models show that compared to quintile 2, the risk of incident dementia was increased in quintile 4 (Hazard Ratio [HR] = 1.05; CI = 1.01±1.10) and quintile 5 (HR = 1.14; CI = 1.09±1.20). In cohort participants ≤60 years old, the risk of incident dementia was increased in quintile 4 (HR = 1.29; CI = 1.12±1.49) and 5 (HR = 1.45; CI = 1.26±1.68). In participants > 60 years old, the risk was not significant in quintile 4, and was attenuated in quintile 5 (HR = 1.10; CI = 1.05± 1.16). Formal interaction analyses showed that the association between phosphorous and dementia was more pronounced in those younger than 60, and attenuated in those older than 60 (P for interaction was 0.004 and <0.0001 in quintiles 4 and 5; respectively). We conclude that higher serum phosphorous is associated with increased risk of incident dementia. This association is stronger in younger cohort participants. The identification of serum phosphorous as a risk factor for incident dementia has public health relevance and might inform the design and implementation of risk reduction strategies.