What is the correct spelling dietitian or dietician?

Answer 1

If you mean the type of person that tells you what to have in your diet then it's dietician.

Answer 2

Dietician or Dietitian are both correct.

Depending on your English locale, and whether you choose to spell with a C or a T. The International Dietetics Association is on-the-fence on the spelling issue.

T is more common within the profession, but C is being used increasingly because of computer spell checkers and the increased widespread use of the profession within society. Only time will determine which letter wins the spelling battle. If you use change velocity, C will win. If you use the profession base, T will win.