Antonyms of adj unnecessary

1 sense of unnecessary

Sense 1
unnecessary (vs. necessary), unneeded -- (not necessary)

necessary (vs. unnecessary) -- (absolutely essential)
        => essential, indispensable -- (absolutely necessary; vitally necessary; "essential tools and materials"; "funds essential to the completion of the project"; "an indispensable worker")
        => incumbent -- (necessary (for someone) as a duty or responsibility; morally binding; "it is incumbent on them to pay their own debts")
        => needed, needful, required, requisite -- (necessary for relief or supply; "provided them with all things needful")
        => obligatory -- (required by obligation or compulsion or convention; "he made all the obligatory apologies")

Similarity of adj unnecessary

1 sense of unnecessary

Sense 1
unnecessary (vs. necessary), unneeded -- (not necessary)
       => excess, extra, redundant, spare, supererogatory, superfluous, supernumerary, surplus -- (more than is needed, desired, or required; "trying to lose excess weight"; "found some extra change lying on the dresser"; "yet another book on heraldry might be thought redundant"; "skills made redundant by technological advance"; "sleeping in the spare room"; "supernumerary ornamentation"; "it was supererogatory of her to gloat"; "delete superfluous (or unnecessary) words"; "extra ribs as well as other supernumerary internal parts"; "surplus cheese distributed to the needy")
       => gratuitous, needless, uncalled-for -- (unnecessary and unwarranted; "a strikers' tent camp...was burned with needless loss of life")
       => inessential -- (not absolutely necessary)
       => spare -- (kept in reserve especially for emergency use; "a reserve supply of food"; "a spare tire"; "spare parts")
          Also See-> inessential#1, unessential#1