A dietitian, medical dietitian, or dietician is an expert in identifying and treating disease-related malnutrition and in conducting medical nutrition therapy, for example designing an enteral tube feeding regimen or mitigating the effects of cancer cachexia. Many dietitians work in hospitals and usually see specific patients where a nutritional assessment and intervention has been requested by a doctor or nurse, for example if a patient has lost their ability to swallow or requires artificial nutrition due to intestinal failure. Dietitians are regulated healthcare professionals licensed to assess, diagnose, and treat such problems. In the United Kingdom, dietitian is a 'protected title', meaning identifying yourself as a dietitian without appropriate education and registration is prohibited by law.

A registered dietitian (RD) (UK/USA) or registered dietitian nutritionist (RDN) (USA) meets all of a set of special academic and professional requirements, including the completion of a bachelor's and/or master's degree in nutrition and dietetics (or equivalent). One or more internships (USA) or clinical placements (UK) must also be completed. These may be allocated and monitored by the university as part of the structured degree programme (UK) or may be applied for separately (USA).

Roughly half of all RD(N)s hold graduate degrees and many have certifications in specialized fields such as nutrition support, sports, paediatrics, renal, oncological, food-allergy, or gerontological nutrition. Although assessment priorities differ depending on the specialist area, a patient's medical and surgical history, biochemistry, diet history, eating and exercise habits usually form the basis of assessment. The RD(N) negotiates a treatment plan with the patient which may include prescriptions, and follow-up visits often focus on maintenance and monitoring progress.

Most RDs work in the treatment and prevention of disease (administering medical nutrition therapy, as part of medical teams), often in hospitals, health-maintenance organizations, private practices, or other health-care facilities. In addition, many registered dietitians work in community and public-health settings, and/or in academia and research. A growing number of dietitians work in the food industry, journalism, sports nutrition, corporate wellness programs, and other non-traditional dietetics settings.