The two parties signed a merger agreement, along with a 10-point "ideological declaration," at a right-wing convention in the south-central town of Przysucha on Saturday, state news agency PAP reported.
Conservative leader Jarosław Kaczyński told the convention that the merger "sends a positive message to society that the national patriotic camp is uniting."
Sovereign Poland politician Patryk Jaki said the decision to join forces was driven by a desire to be "part of a concerted effort to fight for the country's future."
The newly united opposition pledged to intensify efforts to regain power.
The PiS party, which governed Poland from 2015 to last year, announced plans for an active campaign ahead of next year's presidential election to maintain control of the presidency.
Meanwhile, Poland’s main governing party, the centrist Civic Platform (PO), reviewed its first 10 months in power and outlined its future policies at a convention in Warsaw on Saturday.
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Source: IAR, PAP