Abnormal plasma concentrations of potassium in the form of hyper- and hypokalemia are frequent among hospitalized patients and have been linked to poor outcomes. In this study, we examined the prevalence of hypo- and hyperkalemia in patients admitted with a fractured hip as well as the association with 30-day mortality in these patients. A total of 7293 hip fracture patients (aged 60 years or above) with admission plasma potassium measurements were included. Data on comorbidity, medication, and death was retrieved from national registries. The association between plasma potassium and mortality was examined using Cox proportional hazards models adjusted for age, sex, and comorbidities. The prevalence of hypo- and hyperkalemia on admission was 19.8% and 6.6%, respectively. The 30-day mortality rates were increased for patients with hyperkalemia (21.0%, p < 0.0001) compared to normokalemic patients (9.5%), whereas hypokalemia was not significantly associated with mortality. After adjustment for age, sex, and individual comorbidities, hyperkalemia was still associated with increased risk of death 30 days after admission (HR = 1.93 [1.55–2.40], p < 0.0001). After the same adjustments, hypokalemia remained non-associated with increased risk of 30-day mortality (HR = 1.06 [0.87–1.29], p = 0.6). Hyperkalemia, but not hypokalemia, at admission is associated with increased 30-day mortality after a hip fracture.