An enterprise master patient index or enterprise-wide master patient index (EMPI) is a patient database used by healthcare organizations to maintain accurate medical data across its various departments. Patients are assigned a unique identifier, so they are represented only once across all the organization's systems. Patient data can include name; gender; date of birth; race and ethnicity; social security number; current address and contact information; insurance information; current diagnoses; and most recent date of hospital admission and discharge (if applicable).

EMPIs are intended to ensure patient data is correct and consistent throughout the organization regardless of which system is being updated. Non-healthcare organizations also face similar issues maintaining customer records across different departments.

Many software vendors use EMPI and MPI (master patient index) synonymously, because an MPI is only workable if it is used by all software applications across an entire enterprise; that is, "master" implies enterprise-wide scope.

EMPIs use match engines along with the technique of referential matching to more easily identify duplicate patient records.