It could be construed that way, but it depends on what is said after it.
If I said: Needless to say, two plus two is four you wouldn't find it condescending, because generally it is needless to say that two plus two is four.
If I said: Needless to say, cribellate spiders have no need for glands producing hygroscopic adhesive.
You might, depending on your knowledge of spiders, disagree with the first clause and thus find it a little condescending.
It is up to you to judge your potential audience and tailor your writing to them.
english.stackexchange.com/users/None/
No, it just indicates that the writer expects the fact that follows to not be surprising to the reader.
The Cambridge Dictionary Online describes the phrase as: Needless to say: as you would expect; added to, or used to introduce, a remark giving information which is expected and not surprising.
Examples 1)
The life and adventures of Martín Chuzzlerwit (Charles Dickens, 1844)
It has been rumoured, and it is needless to say
the rumour originated in the same base quarters , that a certain male Cbuzzlewit, whose birth must be admitted to be involved in some obscurity, was of very mean and low descent.
2)
The Complete Tales of Uncle Remus
(Joel Chandler Harris, Richard Chase, 2002)
No typographical device could adequately describe Daddy Jack's imitation of the flushing of a covey of partridges, or quail; but it is needless to say that it made its impression upon the little boy.
english.stackexchange.com/users/212/b.roth
I find it is quite the opposite.
If you were to neglect to qualify an obvious statement with something like "needless to say", or "as you know", the reader might then feel like he is being condescended to because you are telling him/her information
he/she already knows.
By adding the qualifier, you establish some common ground, in essence saying "I know that you know what I'm telling you already, and I don't want to insult your intelligence, so please don't be offended."
english.stackexchange.com/users/4489/Ray
It is not condescending at all.
It is just a different set of words used to convey the same meaning as '...
and hence it follows that ....' If it were condescending, then the phrase '...
last but not the least ...' would also be condescending to the person who is addressed last in some list.
Does putting that word 'least' in there mean that there is someone in the room who thinks that person is the 'least' ?
Of course not !
It is just an expression used instead of saying '...
it is thus obvious that...', '... it can thus be deduced that...' , '...it implies that...' Needless to say, needless to say is not condescending at all.
english.stackexchange.com/users/3982/Arjun J Rao
In addition to other answers, I want to add:
In formal writing, that phrase should be avoided.
If it's needless to say, why are you saying it?
If you want to be concise, everything you say should serve a purpose.
At best, it sounds like extra fluff; unless you're writing in a genre in which that fluff is ok.
english.stackexchange.com/users/144232/Cameron
Isn't the term a bit condescending?
Does "needless to say" convey to the reader that he is ignorant?
english.stackexchange.com
<word-choice><meaning>
english.stackexchange.com/questions/11017
english.stackexchange.com/users/1075/Shaihi
The term "needless to say" shouldn't necessary be construed in this way, but it depends on what is said after it. Generally, this phrase simply indicates that the writer expects the fact that follows not to be surprising to the reader. Ultimately, it is up to you to judge your potential audience and tailor your writing to them.  It could be construed that way, but it depends on what is said after it. If you are stating a known fact afterwards, it is not condescending. The person speaking should use their discretion when writing and tailor their writing style. The writer is expects the fact that follows to not be surprising to the reader. Cambridge Dictionary Online  describes the phrase as:   Needless to say:  as you would expect;  added to, or used to  introduce, a remark giving information  which is expected and not surprising. In fact, some would say that it is the opposite to condescending. If you were to neglect to qualify an obvious statement with something like "needless to say", or "as you know", the reader might then feel like he is being condescended to because you are telling him/her information. Some believe it should not be used in formal writing as they are just "filler"  words.
The term "needless to say" shouldn't necessary be construed in this way, but it depends on what is said after it. Generally, this phrase simply indicates that the writer expects the fact that follows not to be surprising to the reader. Ultimately, it is up to you to judge your potential audience and tailor your writing to them. This is generally not seen as condescending at all, but is rather a way to establish common ground and note that the information is already known to the listener or reader. However, this should normally be avoided as unnecessary in formal writing.
Needless to say indicates that the writer expects the fact that follows to not be surprising to the reader and is therefore not condescending. However it could be construed that way.  The answer to this question will depend on the specific context, however, "needless to say" is not normally considered to be condescending in general use.
it is thus obvious that...', '... it can thus be deduced that...' , '...it implies that...' Needless to say, needless to say is not condescending at all.
it is thus obvious that...', '... it can thus be deduced that...' , '...it implies that...' Needless to say, needless to say is not condescending at all.
The writer should tailor their work to their audience. It is suggesting the reader already knows the fact. Dictionary definition It means the opposite Do not use in formal writing.
This is generally not seen as condescending at all, but is rather a way to establish common ground and note that the information is already known to the listener or reader. However, this should normally be avoided as unnecessary in formal writing.
The answer to this question will depend on the specific context. In general use, this phrase is not considered to be condescending.