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.

Yahoo said that 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 January 22, the company said. If de Castro, who the technology blog AllThingsD.com said was born in Portugal, 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, in a statement cited de Castro's experience in Internet advertising and his work "structuring and scaling global organizations."

The 37-year-old Mayer, who gave birth to her first child on September 30, also announced the hiring of de Castro with a Twitter message in which she noted that Monday was her first full day back in the office.

Yahoo is one of the world's most-visited online properties, but revenue has declined in recent years amid competition from Google and Facebook Inc. 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.

Mayer has yet to reveal her plan for revitalizing Yahoo, but has made a few key hires, including new finance chief Ken Goldman and marketing head Kathy Savitt, a former Amazon.com executive.

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 October 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.

(Reporting by Alexei Oreskovic; editing by Matthew Lewis and Bob Burgdorfer)

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