Ms Long-Bailey was sacked from Labour's shadow cabinet after sharing an article that "contained an antisemitic conspiracy theory", according to the Labour Party.
She had shared, on Twitter, an interview with actor Maxine Peake - describing her as an "absolute diamond".
In the article, Peake said the tactic of kneeling on the neck used by a US officer charged with killing George Floyd "was learnt from seminars with Israeli secret services".
Sir Keir said he was delighted to appoint Ms Green, the MP for Stretford and Urmston in Greater Manchester.
"Kate has spent a lifetime campaigning against child poverty and educational inequalities. I look forward to working with her in this new role," he said.
Ms Green, who has an OBE, said: "It's a privilege to have been asked to serve as shadow education secretary.
"The coronavirus pandemic has had a devastating impact on children's education. I look forward to working with teachers, unions, parents and councils to help ensure we get our children back in school as soon as possible."
Ms Green was formerly the chair of the London Child Poverty Commission and was elected as an MP in 2010.
She has previously spoken out about antisemitism in the Labour Party and chaired Owen Smith's leadership campaign when he unsuccessfully challenged Jeremy Corbyn in 2016.
Several prominent members of what was Mr Corbyn's shadow cabinet have backed Ms Long-Bailey following her sacking.
Ex-shadow chancellor John McDonnell shared a petition calling for the Salford MP to be reinstated, claiming "criticism of practices of Israeli state is not antisemitic".
But Labour MP Dame Margaret Hodge, who is Jewish, tweeted: "This is what a change in culture looks like. This is what zero tolerance looks like. This is what rebuilding trust with the Jewish community looks like."