Oxford has the following to say:
fiction • noun 1 prose literature, especially novels, describing imaginary events and people. 2 invention as opposed to fact. 3 a false belief or statement, accepted as true for the sake of convenience.
— DERIVATIVES fictional adjective fictionality noun fictionalize (also fictionalise) verb fictionally adverb fictionist noun.

fictitious /fiktishss/ • adjective 1 not real or true, being imaginary or invented. 2 referring to the characters and events found in fiction.

Odd that fictitious is not listed among the derivates above and that fictional does not rate its own entry. They seem to mean the same thing though.

My native speaker instincts would have said to use "fictional character" for a character from fiction, and fictitious as an adj. meaning not true "His story is completely fictitious." But it appears I am wrong.

The two words are pretty much interchangable.

However, fictitious implies that although something is not completely factual, it might be based on elements of truth (see "apocryphal"). For instance, the gospel story of Jesus feeding the five thousand might be considered fictitious in terms of exactly how many people got fed and exactly how the food was multiplied. Yet the underlying story of people being nourished probably happended in some form.

A fictional story like Harry Potter is solely the contrivance of imagination. There really is no Hogwarts school and no Harry Potter (at least not one who is a wizard).

I checked with Webster's, fictional= related to, characterized by, suggestive of fiction.
fictitious= imaginary, assumed, feigned, simulated.
According to the comprehensive explanation of fictitious, this seems to be the more commonly used term. See your Google hits.

Bin kein Native Speaker und mag völlig falsch liegen, aber ist es nicht so, dass sich "fictional" nur auf Literatur und ähnliches bezieht?

"fiktiv" im Sinne von einer Annahme / Hypothese oder gar einer gewissen Willkür ist, glaube ich, immer "fictitious". Also "We tested using fictitious data", nicht "fictional data".

Oder ist das Quatsch?

Fictional character: A person from a novel, play, etc.; clearly from literature, e.g. Harry Potter
Fictitious character: A person who doesn't exist in reality, but someone claims he does. e.g. "I didn't do it; it wasn't me; it was my evil twin brother."

Main Entry: fic·ti·tious
Pronunciation: fik-'ti-sh&s
Function: adjective
Etymology: Latin ficticius artificial, feigned, from fictus
1 : of, relating to, or characteristic of fiction : IMAGINARY
2 a : conventionally or hypothetically assumed or accepted <a fictitious concept> b of a name : FALSE, ASSUMED
3 : not genuinely felt
- fic·ti·tious·ly adverb
- fic·ti·tious·ness noun
synonyms FICTITIOUS, FABULOUS, LEGENDARY, MYTHICAL, APOCRYPHAL mean having the nature of something imagined or invented. FICTITIOUS implies fabrication and suggests artificiality or contrivance more than deliberate falsification or deception <fictitious characters>. FABULOUS stresses the marvelous or incredible character of something without necessarily implying impossibility or actual nonexistence <a land of fabulous riches>. LEGENDARY suggests the elaboration of invented details and distortion of historical facts produced by popular tradition <the legendary exploits of Davy Crockett>. MYTHICAL implies a purely fanciful explanation of facts or the creation of beings and events out of the imagination <mythical creatures>. APOCRYPHAL implies an unknown or dubious source or origin or may imply that the thing itself is dubious or inaccurate <a book that repeats many apocryphal stories>.

Ich würde auch wie Sophil sagen, dass im Englischen derselbe Unterschied wie im Deutschen gemacht wird:
fictional = fiktional (vor allem im literarischen Sinne)
fictitious = fiktiv / fingiert (zu fingieren: sich ausdenken, erfinden)

Thanks everybody for your comments!

LEO kennt bis jetzt das deutsche Wort "fiktional" genausowenig wie das englische "fictional"...

Der Duden sagt:

fiktiv - nur angenommen; erdacht, erdichtet, frei erfunden
fiktional - auf einer Fiktion beruhend
Fiktion - Einbildung, Annahme

Der Unterschied ist, wenn, dann auf jeden Fall ein sehr feiner. Ich denke, man kann in den meisten Fällen sowohl im Englischen wie auch im Deutschen beide Varianten verwenden.

I fail to see how one can confuse one of these with the other, especially considering that the quoted dictionaries make a useful differentiation between them.

fictional - coming from fiction, or a work of fiction
the fictional character Harry Potter
fictitious - imaginary, not real
the fictitious evil brother mentioned above.

the distinction is clear, welcome, and useful.