Verizon now says data from all Yahoo users was affected by a 2013 data hack.
The company announced Tuesday that its previous disclosure of an estimated one billion accounts hit by the theft was incorrect and that all three billion accounts were impacted.
The extent of the breach was discovered during the integration of the two firms, Verizon said, when it "obtained new intelligence."
Email notifications requesting password changes are being sent to the additional user accounts. Verizon said passwords in clear text, payment card data and bank information was not among the information stolen.
Verizon purchased Yahoo in a $4.5 billion deal that closed in June. The deal is aimed at ushering in a new management team to attempt to wring more advertising revenue from one of the internet's best-known brands.
The deal's price was cut by $350 million after Yahoo's initial revelations of the data breaches. The revised transaction terms called for Yahoo and Verizon to share equally any future legal costs resulting from the data breaches. Yahoo would be solely responsible for liabilities stemming from shareholder lawsuits and any investigations by securities regulators.
The Associated Press contributed to this report.