SANFORD, Fla. — George Zimmerman, the neighborhood watch volunteer who fatally shot Trayvon Martin, an unarmed black teenager, igniting a national debate on racial profiling and civil rights, was found not guilty late Saturday night of second-degree murder. He was also acquitted of manslaughter, a lesser charge.

After three weeks of testimony, the six-woman jury rejected the prosecution’s contention that Mr. Zimmerman had deliberately pursued Mr. Martin because he assumed the hoodie-clad teenager was a criminal and instigated the fight that led to his death.

Mr. Zimmerman said he shot Mr. Martin on Feb. 26, 2012, in self-defense after the teenager knocked him to the ground, punched him and slammed his head repeatedly against the sidewalk. In finding him not guilty of murder or manslaughter, the jury agreed that Mr. Zimmerman could have been justified in shooting Mr. Martin because he feared great bodily harm or death.

The jury, which had been sequestered since June 24, deliberated 16 hours and 20 minutes over two days. The six female jurors entered the quiet, tense courtroom, several looking exhausted, their faces drawn and grim. After the verdict was read, each assented, one by one, and quietly, their agreement with the verdict.