Tillerson will hold talks over lunch with European Union foreign ministers and the bloc's diplomatic chief Federica Mogherini, before a two-day NATO meeting set to focus on North Korea's missile programme and concerns over perceived hostility from Russia.
But his visit comes against a difficult backdrop - a rift with President Donald Trump has led to reports he could be replaced within weeks, calling into question his authority to speak for Washington.
NATO chief Jens Stoltenberg said yesterday it was vital international powers worked together to tackle the North Korean crisis, after Pyongyang tested a long-range missile it said could hit anywhere in the continental United States.
"Last week's launch of an intercontinental ballistic missile showed that all allied nations could be within range," Stoltenberg said yesterday.
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"The whole world needs to apply maximum pressure on North Korea in order to achieve a peacefully negotiated solution."
The EU has been ramping up economic sanctions on the North in a bid to force it to the negotiating table - but with no success so far.
But if the US and EU can present a unified front on North Korea, the deal with Iran to end the Islamic republic's nuclear programme in return for the lifting of sanctions is more problematic.
European powers are keen to maintain the deal and Mogherini last month travelled to Washington to lobby US lawmakers not to withdraw from the agreement.
"Preserving the nuclear deal with Iran and its full implementation is a key security priority for Europe," Mogherini said on Friday.
Tillerson's Brussels visit comes at the start of a European tour taking in Paris and Vienna for the 57-member Organization for Security and Cooperation in Europe.
Yesterday Stoltenberg gave his backing to Tillerson's efforts in tackling the North Korean crisis - an issue where Trump has publicly criticised his top diplomat, saying he was "wasting his time" pursuing contacts with Pyongyang.
"Secretary Tillerson has played a key role, both in sending the message of deterrence, the unity and the resolve of the whole alliance, but also when it comes to the need for continuing to work for a peaceful solution," Stoltenberg said.