President Andrzej Duda greenlighted the measure after it passed in a 62-1 vote with no abstentions in the upper house of parliament at the end of last month.
The legislation was earlier overwhelmingly approved by the lower chamber of Poland's bicameral parliament.
Under the new rules, personal income tax will be reduced for everyone earning less than PLN 85,528 (USD 22,537, EUR 20,087) a year.
The change is expected to come into force on October 1, benefiting 25 million taxpayers, public broadcaster Polish Radio’s IAR news agency has reported.
The Polish president earlier this summer signed into law a plan by the country’s ruling conservatives to make people under 26 exempt from paying personal income tax.
Poland is due to hold parliamentary elections on October 13.
The governing conservative Law and Justice (PiS) party is seeking a second term in power after a landslide win in 2015.
Source: IAR, PAP