The deployment will raise the total number of active-duty forces supporting Customs and Border Protection agents there to approximately 4,350, according to the department.
The additional units will be deployed for 90 days.
The Pentagon first approved the high-profile deployment of active-duty US troops to the Mexico border in October, before the US midterm congressional elections.
The move was embraced by Donald Trump’s supporters, including fellow Republicans in Congress.
Acting Defense Secretary Patrick Shanahan approved the deployment on 11 January, according to the statement.
US Representative Adam Smith, chairman of the House of Representatives Armed Services Committee, first disclosed plans to send more troops on Thursday.
Mr Smith criticised senior Pentagon officials for failing to disclose the information during a hearing before his committee in Congress on the issue just two days earlier.
Additional reporting by Reuters
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