"Intelligent" means "having a keen mind." For example, it means being able to solve difficult problems.
"Intellectual" means "living a life devoted mostly to things of the mind." It means spending most of one's time writing, thinking, and talking about ideas.
"Intellectual" can be an adjective ("Susan Sontag was a very intellectual person") or a noun ("Susan Sontag was an intellectual.")
Critics, philosopher, economist, and professors are examples of intellectuals.
For example, Bill Gates is highly intelligent, but since he spent his life building a business, I would not call him an intellectual. Doctors, astronauts, and generals are usually intelligent, but not intellectual.
It is probably correct to say that being "intellectual" does imply being fairly intelligent.