Pc Joseph Harrington is said to have kicked a 15-year-old boy to the ground before kneeing him after he had been taken to an east London police station last August.
The allegation was investigated by the Independent Police Complaints Commission and evidence passed to the Crown Prosecution Service.
In February this year, the CPS decided against any charges after finding there was "insufficient evidence" to charge Pc Harrington.
But last month a review was ordered after "concerns were raised".
Alison Saunders, chief crown prosecutor for CPS London, said it was now complete and that there was "sufficient evidence" to charge the officer with assault occasioning actual bodily harm.
She added: "Occasionally, a new look at a prosecution decision shows that the wrong view of the evidence was taken. That is regrettable. In this case, the conclusion originally reached was not tenable on the available evidence.
"When a review shows a previous decision not to prosecute is clearly wrong, it is open to the CPS to rectify that error by reconsidering the prosecution decision at Chief Crown Prosecutor level.
"As Chief Crown Prosecutor for London, I have taken the decision in this case that not only is there sufficient evidence to provide for a realistic prospect of conviction, and that a prosecution is required in the public interest, but that a prosecution is necessary in order to maintain confidence in the criminal justice system.
"That is the test I must apply under the Code for Crown Prosecutors when reinstituting a prosecution. It is clear that the allegation of using excessive force on a handcuffed 15 year old in custody is a serious matter."
