Abu Sittah was due to speak at a Palestinian conference in Berlin on Friday.
The surgeon said he had been invited to the conference to speak "about my work in Gaza hospitals".
"The German government has forcibly prevented me from entering the country," he posted on X.
Middle East Eye has asked the German foreign ministry and Abu Sittah for comment.
Since leaving Gaza in late November, the doctor has been raising awareness about the impact of Israel's war, which has killed more than 33,000 Palestinians.
In the initial weeks after Israel began its assault, Abu Sittah was the unofficial English-language representative of Palestinian doctors and surgeons treating Palestinians wounded by Israeli attacks.
He accused the Israeli military of using white phosphorus, which is illegal in built-up and populated areas like Gaza, and deliberately targeting children.
In an interview with MEE after he left Gaza, Abu Sittah said medics were using household items to treat patients because of an Israeli-imposed blockade on medical equipment entering the enclave.
"Eventually, everything was running out. Initially, we replaced the antiseptic solution with washing-up liquid and vinegar,” Abu Sitta said.
"Then it ended up being morphine and having to do procedures without any anaesthetic. The situation was medieval-like.”
Berlin has been a key supplier of armaments to Israel in its war on Gaza and has also been one of its main advocates.
Germany is currently subject to an International Court of Justic (ICJ) case filed by Nicaragua, which accuses it of “facilitating” Israeli genocide in Gaza. The ICJ is also considering a case filed by South Africa accusing Israel of genocide.
Abu Sittah on Friday said Germany was "silencing a witness to genocide before the ICJ", which he said, "adds to Germany's complicity in the ongoing massacre".
Since the start of the war six months ago, Germany has justified its support for Israel by claiming it has a special duty to protect it because of the legacy of the Holocaust, in which the Nazis killed six million Jews.
In October, Federal Chancellor Olaf Scholz said: "Israel’s security is part of Germany’s raison d’état."
At home, the German government has stifled expressions of solidarity with the plight of Palestinians.
Funding has been withdrawn from cultural institutions that express sympathy for the victims of Israel’s assault in Gaza and there have been restrictions placed on public demonstrations of support.
Some local officials within the state have demanded foreigners seeking naturalisation recognise the legitimacy of the Israeli state before being granted citizenship.
In March, Berliner Sparkasse Bank, a state-owned bank, froze the accounts of Jewish Voice, a Jewish group advocating for the rights of Palestinians, and demanded the organisation hand over its membership lists.
"In 2024, Jewish money is once again being confiscated by a German bank: Berliner Sparkasse," Jewish Voice said in a statement on its social media platforms.