Rajoy initially announced he would appear in the parliament during a press conference on Monday.
The Spanish leader said he would appear to give his "version" of events in relation to the slush fund scandal rocking the PP.
Former PP treasurer Luis Bárcenas alleges he operated a second set of books for the Spain's ruling PP party over many years.
Bárcenas — current being held in custody without bail for his alleged involvement in another corruption scandal — testified in court last Monday that Rajoy received €25,000 ($33,000) in cash in 2010.
Rajoy has denied any wrongdoing and has repeatedly rejected calls for him to step down over the slush fund scandal.
And despite opposition parties — led by Spain's main opposition socialist PSOE party — calling for the Spanish PM to appear in parliament to explain the alleged illegal funding scandal, Rajoy had until today kept silent on the issue.
Now, however, Rajoy has said he will indeed appear in parliament, on August 1st at 9am.
On Sunday a poll published in the El Mundo daily, showed 89.1 percent of people surveyed thought Mariano Rajoy should answer allegations that he accepted secret payments from the PP.
Only 2.4 percent of poll respondents didn't think Rajoy needed to offer up that explanation.