Thomas Bossert will become an assistant to the president for homeland security and counterterrorism.
A statement from Trump's transition team said Bossert will advise the president on issues related to homeland security, counterterrorism and cybersecurity, and coordinate the Cabinet's process for making and executing policy in those areas.
The president-elect also appointed one of his main advisers on US-Israel relations during the campaign and transition as special representative for international negotiations.
The homeland security position notably "is being elevated and restored to its independent status alongside the national security adviser," the statement said. Policymakers have long debated whether such national security jobs should operate independently from the White House.
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Bossert will work closely with Trump's pick for national security adviser, retired Lt Gen Michael Flynn.
Bossert is currently president of the risk management consulting firm CDS Consulting. He previously served as deputy assistant to the president for homeland security under Bush.
Trump's influential son-in-law, Jared Kushner, will have a direct line to the president as well.
Trump has long stoked rivalries among his staff in business and during his presidential campaign. But some critics fear doing so in the White House could sow confusion and slow decision-making.
Discussing the appointment of Greenblatt, Trump said he "has a history of negotiating substantial, complex transactions on my behalf," and has the expertise to "bring parties together and build consensus on difficult and sensitive topics."
The billionaire-turned-president-elect is spending the holidays at his Florida resort, where he's held a steady stream of meetings with senior staffers, advisers and business executives. A number of key posts in his administration remain; his transition team says he will fill those positions in the coming days.