In the letter Bana calls on the Prime Minister to help Syrian children by sending food and medicine to civilians in the war-ravaged country.
Bana shared a picture of the hand-written letter on Twitter, tagging Mrs May.
In the letter she tells the Prime Minister: “I am looking for help for the suffering people of Syria. Can you send them medicine, doctors, water and milk?”
“Have you seen the young children who are weak and dying because of hunger?” she asks, continuing “I have seen them. They live if we give only food but not one cares.”
Bana goes on: “They live if we give only food but no one cares. I am very sad. Promise me you send them food and medicine now please. Do not forget them.”
She signs off the letter saying “love to you, Dear Theresa May.”
The seven-year-old, who has amassed 363,000 followers on Twitter, documented the war in Aleppo by posting updates with the help of her mother Fatehmah.
Bana escaped the bombarded city with her family in December and is now a refugee in neighbouring Turkey.
It is not the first time she has written to world leaders to raise the plight of Syrians.
Last year she wrote to the then US President-elect, Donald Trump, asking for his help.
In the days before Mr Trump’s inauguration she wrote: “I know you will be the president of America, so can you please save the children and people of Syria?”
She has also written to Syrian President Bashar al-Assad and his Russian ally Vladimir Putin imploring them to “stop the bombing now” to “save the children of Syria”.
She also told them to “go to jail for killing my friends.”
Bana and her younger brother also met Turkey’s President Recep Tayyip Erdogan in Ankara after they left Syria.
It is not known if Mrs May has responded to Bana's letter.