Starmer is heading over the English Channel for talks focused on defence and security co-operation and will also meet Nato Secretary General Mark Rutte.
The trip is part of what he calls a "reset" between the UK and the European Union.
The government has promised the UK will not re-join the EU's Single Market or Customs Union, or sign up to freedom of movement.
But ministers do want what they see as a better relationship on defence and security, crime and trade.
They hope to sort this out by the spring and finalise it at a UK-EU summit, possibly in April or May.
Leaders of the European Union's 27 member states are gathering for what is described as an "informal retreat" at the Palais d'Egmont in Brussels - a 16th century palace in heart of the Belgian capital.
The backdrop is clear: the ongoing war in Ukraine and the return of Donald Trump to the White House.
The EU acknowledges it needs to take greater responsibility for its own defence – a key and recurring demand of President Trump, as he threatens the bloc with import taxes or tariffs.
The prime minister said: "President Trump has threatened more sanctions on Russia and it's clear that's got Putin rattled. We know that he's worried about the state of the Russian economy.
"I'm here to work with our European partners on keeping up the pressure, targeting the energy revenues and the companies supplying his missile factories to crush Putin's war machine.
"Because ultimately, alongside our military support, that is what will bring peace closer."
