Maria Dolores de Cospedal's name has appeared on a list of top party members who allegedly received tens of thousands of euros in secret cash payments from the slush fund operated by former party treasurer Luis Barcenas.
The scandal first broke in January when a newspaper published copies of account ledgers purportedly showing irregular payments to top party members including Prime Minister Mariano Rajoy who has led the conservative PP since 2004.
Barcenas told National Court Judge Pablo Ruz, who is leading the investigation, that he made secret cash payments to Rajoy and Cospedal between 2008 and 2010 while they were key opposition leaders.
Rajoy has denied that he or other party figures received illegal payments and has said he will not step down over the affair.
Ruz has summoned Cospedal to testify as a witness on August 14, the judicial source told AFP.
He also summoned her two predecessors in the post of PP secretary general, Javier Arenas and Francisco Alvarez Cascos, to testify on August 13, the source added.
Cospedal, 47, is also the head of the regional government of Castilla-La Mancha.
Pressure on Rajoy over the case has mounted after Barcenas was jailed last month while under investigation for running the slush fund and having huge secret Swiss bank accounts.
The prime minister will address parliament over the issue on Thursday.