After 20 months of negotiations, the 27 leaders gave the deal their blessing after less than an hour's discussion.
They said the deal - which needs to be approved by the UK Parliament - paved the way for an "orderly withdrawal".
But getting the backing of MPs would be "challenging", Foreign Secretary Jeremy Hunt has told the BBC.
The UK is scheduled to leave the EU on 29 March 2019.
The EU officially endorsed the terms of the UK's withdrawal during a short meeting in Brussels, bringing to an end negotiations which began in March 2017.
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