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Introduction: The present study explored the relationship between hypomagnesemia and outcome in community-acquired pneumonia (CAP) patients with type 2 diabetes mellitus (T2DM). Methods: The study was a retrospective cohort study, conducted on adult CAP patients with T2DM at The First Hospital of Qinhuangdao between January 2015 and December 2018. The primary outcome was all-cause deaths. Results: In total, 480 CAP inpatients with T2DM were enrolled, and 71 patients (14.8%) had hypomagnesemia on admission. After one month and three months of follow-up, the hypomagnesemia group had a higher mortality rate than the normal magnesium group ( p<0.05). After six months of follow-up, the mortality rate remained higher in the hypomagnesemia group, however, this was not statistically significant (χ2=2.799, p=0.094). After 12 months and 24 months of follow-up, the mortality rates were similar between the hypomagnesemia and normal magnesium groups ( p>0.05). Based on multiple logistic regression analysis, hypomagnesemia was an independent risk factor for one-monthmortality(OR=3.858,95% CI: 1.637~9.088, p=0.002), three-month mortality (OR=3.083, 95% CI: 1.434~6.627, p=0.004) and six-month mortality (OR=2.551, 95% CI: 1.209~5.383, p=0.014). Conclusions: Hypomagnesemia is common in CAP inpatients with T2DM. Moreover, in these patients, hypomagnesemia at admission is associated with increased mortality in the short-term but not the long-term.