In what is the first time the council has balloted members on a local issue, at least 160 jobs are threatened. The council has sent compulsory redundancies notices to the first wave of 20 staff this week.
Unison has 3,500 members and an all-out strike could hit services across all departments.
Front-line services are under threat of cuts include child protection and care homes.
Unison branch secretary Alex Knutsen said: "In ten years there has never been a full ballot on a local issue. The numbers of jobs being cut is a significant number for an authority of this size.
He said that the union would "fight to defend members livelihoods".
"We have never faced compulsory redundancies and we feel that the council has moved to issue them very rapidly."
Last summer the unions claim they urged the council to freeze vacancies to limit job cuts but claim this was ignored.
The council was unavailable for comment.
Meanwhile, councillors at the council are due to consider proposals to increase charges by up to 20% for the in-demand beach huts. The rises, which are the first in three years, will take the cost up to£430 in some areas although the distinctive privately owned wood huts in Hove will see a 6% rise to£270.