'Secretary of Homeland Security Kirstjen Nielsen will be leaving her position, and I would like to thank her for her service,' Trump tweeted on Sunday evening.
He announced U.S. Customs and Border Protection Commissioner Kevin McAleenan will be acting secretary.
'I am pleased to announce that Kevin McAleenan, the current U.S. Customs and Border Protection Commissioner, will become Acting Secretary for @DHSGov. I have confidence that Kevin will do a great job!,' he wrote.
Hours after announcing Nielsen's resignation, President Trump on Sunday took credit for the large number of arrests at the border while threatening to close the frontier with Mexico if Democrats in Congress do not agree to reform the immigration system.