Mr Netanyahu called on right-wing members of parliament to block the coalition from taking office.
Eight opposition parties reached an agreement to work together to form a new government late on Wednesday.
But the group, from across Israel’s political spectrum, still needs parliamentary backing to take office.
In his first comments since the coalition was announced, Mr Netanyahu urged members of the Knesset (parliament) “elected by votes from the right” to oppose the coalition.
In a post on Twitter, he criticised them as “left-wing” and “dangerous”. He has previously called the proposed new government the “fraud of the century”, saying it endangered the state and people of Israel.
Observers have already noted that Mr Netanyahu – who failed to form his own coalition despite his Likud party winning the most seats in the March vote – is likely to try to prevent the group getting the support it needs.
The coalition, formed by Yair Lapid, leader of the centrist Yesh Atid party, needs the parliamentary vote before being sworn in. It is unclear how long this will take, and there is still a chance this newly formed alliance could be upended by defections.
Mr Lapid, whose party came second in the election, called President Reuven Rivlin to let him know the agreement had been reached on Wednesday night.
He pledged to form a government which would “work in the service of all Israeli citizens… respect its opponents and do everything in its power to unite and connect all parts of Israeli society”.