Antonyms of adj compulsory

1 sense of compulsory

Sense 1
compulsory, mandatory, required -- (required by rule; "in most schools physical education is compulsory"; "attendance is mandatory"; "required reading")

INDIRECT (VIA obligatory) -> optional -- (possible but not necessary; left to personal choice)

Similarity of adj compulsory

1 sense of compulsory

Sense 1
compulsory, mandatory, required -- (required by rule; "in most schools physical education is compulsory"; "attendance is mandatory"; "required reading")
       => obligatory (vs. optional) -- (morally or legally constraining or binding; "attendance is obligatory"; "an obligatory contribution")