Leaders of the centre-right CDC party and left-wing ERC sealed a pact at a meeting on Tuesday, agreeing to run on a joint ticket on September 27, sources in both parties told AFP.
Spain's conservative national government fiercely opposes independence for the rich northeastern region, which wants to follow Scotland's example by voting on its political future.
The parties want the regional election to serve as a de facto referendum by running on a joint ticket, campaigning on the single issue of independence.
They agreed that if they jointly win a majority, they will form a coalition government led by current regional president Artur Mas that will aim to achieve independence within 18 months.
An ERC source said Mas's CDC and ERC leader Oriol Junqueras would let a Green politician and two leaders of pro-independence civil groups head the electoral list to push the independence agenda forward.
The CDC currently governs Catalonia, a region with numerous pro- and anti-independence parties, but it split in June from its longtime coalition partner UDC in disagreement over the path to sovereignty.
Mas tried to hold a referendum on independence last November but that was blocked by the national courts after the Spanish government appealed against it.
Instead Mas held a symbolic, non-binding vote in which he said a majority of participants backed independence.
A Catalan government poll published on July 3rd indicated that support for independence in Catalonia had slipped from 44.1 to 42.9 percent, with 50 percent of voters opposed to it.