De Castro, who will oversee Yahoo's global sales, operations, media and business development, will be eligible for a whopping $58 million total compensation package, according to an SEC filing by Yahoo on Monday, Reuters reported.
Yahoo said de Castro would receive $600,000 in annual base salary, as well as a $36 million in restricted stock units and stock options as a one-time retention award, among other parts of his compensation package.
De Castro will start at Yahoo by Jan. 22, the company said. If de Castro, a Portuguese national, has not obtained a work visa prior to his official start date he will initially work in London for a Yahoo subsidiary, the company said.
Mayer, a former Google executive who joined Yahoo as CEO in July, cited de Castro's experience in Internet advertising and his work "structuring and scaling global organizations."
Yahoo is one of the world's most-visited online properties, but revenue has declined in recent years amid competition from Google and Facebook. Yahoo has also been beset by internal turmoil that has resulted in a revolving door of CEOs. The surprise appointment of Mayer in July made her Yahoo's third chief in about 12 months.
De Castro is currently vice president of Google's worldwide partners business solutions group, overseeing advertising services for the company's publisher and commerce partners.
Shares of Yahoo, which reports third-quarter results on Oct. 22, were up 7 cents at $15.75 in after-hours trading on Monday. The shares fell 1.3 percent to $15.68 during the regular session.