A specialist registrar is a doctor in the United Kingdom who is receiving advanced training in a specialist field of medicine in order eventually to become a consultant.

After graduation from medical school, they will have undertaken several years of work and training as a pre-registration house officer and senior house officer, and will usually have taken examinations for membership of the Royal College of their speciality. For example, medical registrars will take the MRCP examinations to qualify them to become registrars.

Competition for specialist registrar posts is regarded as tough. The applicant must first get a National Training Number which allows them to enter specialist training, and then secure a post. The number of posts available is strictly proportional to the number of consultants required in a particular speciality, and therefore in the more popular specialities it can often take many attempts to get a post - leading to what is known as the "SHO bottleneck", whereby doctors are stuck at the grade of senior house officer for a number of years. Changes in postgraduate medical training ("Modernising Medical Careers") are underway to alleviate this problem.

Specialist registrars generally stay in post for around five years (more or less depending on the speciality), gaining experience at first in a broad speciality (e.g. general medicine), later specialising in a subspeciality (e.g. cardiology) after which they receive the certificate of completion of specialist training (CCST), and are eligible for promotion to consultant. Specialist registars are encouraged to undertake research in their field, and many choose to do this by means of a PhD or MD.

Medical education | National Health Service