"I have ended the strike," the activist wrote in the letter handed to his family on Tuesday, but dated the day before, shared by his sister Mona Seif.
It was not clear what had prompted his decision. He also said in the note he had received a short letter from his mother as well as an MP3 digital music player he requested she send.
He wrote to his mother: "I want to celebrate my birthday with you on Thursday".
In what was the second letter from the dissident received by his family in two days, Abdel Fattah asked his mother to "bring a cake" to her monthly visit to the Wadi al-Natroun prison, about 100 kilometers northwest of Cairo.
"I'll see you on the visit day and tell you everything then, and we'll get back to long letters after the visit."
His sister, who has been campaigning for his freedom for years, said she welcomed the news with "cautious relief."
"My heart won't really be settled until Thursday when my mother and sister see him with their own eyes."
Abdel Fattah has been leading headlines since UN climate talks began earlier this month in Egypt, which sought to burnish its image by hosting COP27 but has come under fire over its human rights record.
The dissident, who had consumed "only 100 calories a day" for seven months, had escalated his strike to all food and then stopped drinking water as the COP27 climate summit opened on November 6 in the Egyptian resort town of Sharm el-Sheikh.
Several raised his case in bilateral meetings with President Abdel Fattah al-Sisi, notably US President Joe Biden on Friday.
Several speakers at the summit ended with the words "you have not yet been defeated" -- the title of Abdel Fattah's book published while behind bars.
Fears had mounted that prison authorities were force-feeding Abdel Fattah, after his mother was notified he had been put "under medical supervision."
British officials had for months unsuccessfully sought consular access to visit him.
After his family announced the end of the strike Tuesday, Tarek el-Awady -- a member of the recently reactivated presidential pardoning committee -- wrote on Twitter that he "hopes the state will take the necessary measures to quickly pardon" Abdel Fattah and other prisoners.
This report was compiled using data from Reuters and Agence France-Presse