Former deputy prime minister Soraya Saenz de Santamaria, 47, and ex-defence minister Maria Dolores de Cospedal, 52, both threw their hats into the ring to replace Rajoy, who withdrew from politics after his government was toppled last month in a parliamentary confidence vote.
As the PP's general secretary since 2008, De Cospedal ran the party while Saenz de Santamaria took care of the government's day-to-day business and acted as its spokesperson, giving her a high public profile.
The leadership race was blown open on Monday when the head of the regional government of Galicia, Alberto Nunez Feijoo, who is close to Rajoy and had long seen as the favourite to succeed him, ruled himself out.
"I want to be the first woman who heads the PP and I also want to be the first woman who heads the government of Spain," Cospedal said during a speech in the central city of Toledo.
Spain's new Socialist Prime Minister Pedro Sanchez has won widespread praise at home and abroad for naming women to a majority of cabinet posts for the first time in Spain's history as the women's rights movement in the country gains strength.
"If we run a good opposition we will be back in government immediately," Saenz de Santamaria told reporters in Madrid after announcing on Twitter that she would run to lead the PP.
The deadline to declare candidacies for the party's first-ever leadership election is on Wednesday.
So far four other candidates have come forward, including former foreign minister Jose Manuel Garcia Margallo, 73, and Pablo Casado, a 37-year-old party spokesman and rising star whose reputation has taken a hit over questions about how he obtained a masters degree.
The PP's roughly 800,000 members will be able to cast their ballots during the first round of voting on July 5.
If no one wins more than half the ballots, the two top candidates will head to a final vote at a party congress on July 20-21 where only representatives of the PP's provincial chapters will take part.
The new leader will guide the PP through municipal, regional and European elections next year.
The party has been tainted by a string of corruption scandals which led to Rajoy's downfall, and is facing a stiff challenge by new centrist party Ciudadanos.