The judge said that the 15-year-old boy (who can't be named) should be cleared of all charges.
He can now go home - and for him, the trial is over.
Now only two 16-year-old brothers remain on trial for the murder of 10-year-old Damilola.
At the moment the reasons for the judge's decision aren't known.
Last month a 17-year-old boy walked free when the judge said he didn't trust some of the evidence given by a 14-year-old girl.
Damilola bled to death after being stabbed in the leg with a broken bottle in Peckham, London, in November 2000.