Webster’s Unabridged Dictionary gives the same definition for both fictional and fictitious:
of, relating to, characterized by, or suggestive of fiction.
“Fiction,” of course, is anything untrue, whether it’s a lie or a novel. Webster’s gives six definitions for fiction, among them:
1 : the act of creating something imaginary : a fabrication of the mind
2 a : an intentional fabrication : a convenient assumption that overlooks known facts in order to achieve an immediate goal b : an unfounded, invented, or deceitful statement
3 a : fictitious literature (as novels, tales, romances) b : a work of fiction; especially : NOVEL
Making a distinction between fictional and fictitious, however, is both useful and customary.
Fictional tends to be used in talking about fiction in the sense of creative writing:
Alice in Wonderland is a fictional character created by the mathematician Charles Dodgson.
Fictitious tends to carry a negative connotation and is used to denote fiction associated with dishonesty:
The man used a fictitious resumé to obtain the job.
We praise Ken Follett for writing a fictional account of the building of a cathedral, but we condemn a journalist who incorporates fictitious elements in a news story.