Announcing Snap's entry today, PKR de facto leader Anwar Ibrahim said the coalition's first task was to support DAP's candidate for the by-election, which will be announced on April 27.
The Dayak-based Snap, which is one of the oldest parties in Sarawak, was a member of the state BN ruling coalition, and is now an opposition party.
It does not have a seat in the state assembly.
Anwar and top Pakatan leaders have met with Snap representatives in Sibu on Sunday to seal the deal.
In the 2008 general elections, a lack of formal electoral pact resulted in numerous multi-cornered contents involving Snap, DAP and PKR.
With a formal coalition in place, the four opposition parties are now in a better position to ensure that the upcoming state elections will only see straight fights.