Not exactly. A registered dietitian, or RD, can be a nutritionist; but a nutritionist can not call themselves an RD.
The definition and requirements for the term "nutritionist" vary. Some states have licensure laws that define the range of practice for someone using the designation "nutritionist," but in other states, virtually anyone can call him- or herself a "nutritionist" regardless of education or training.
Individuals with the RD credential have fulfilled specific requirements, including having earned at least a bachelor's degree, completed a supervised internship program and passed a registration examination — in addition to maintaining continuing education requirements for recertification.
So, when seeking guidance on nutrition and health, seek out a true "Registered Dietitian". This will ensure expert advise.