The Prime Minister attended a service at St Clement's Church, a stone's throw from the west London high-rise block, and laid a wreath with a handwritten message on Monday evening.
She was accompanied by Fire Minister Nick Hurd, who left a bouquet of flowers.
Afterwards, Mrs May went to the nearby Al Manaar mosque, where she donned a headscarf and joined worshippers observing Ramadan as they broke their fast.
It is understood she spoke directly with survivors during the private evening visit.
Earlier on Monday, the PM revealed in the Evening Standard that she will "always regret" failing to meet survivors on her first visit to the scene of the disaster.
She said those caught up in the tragedy needed to know that people in power understood their despair and admitted it "seemed as though I didn't care".
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