The six-member, all-woman jury deliberated for more than 15 hours over two days before reaching their decision Saturday night.
They had been given the chance to convict Zimmerman of manslaughter but did not do so, despite asking for a clarification of the charge earlier in the evening.
After hearing the verdict, Judge Debra Nelson told Zimmerman he was free to go.
Zimmerman, 29, blinked and barely smiled when the verdict was announced. He could have been convicted of second-degree murder or manslaughter. But the jury of six women, all but one of them white, reached a verdict of not guilty after deliberating well into the night Saturday. The jurors considered nearly three weeks of often wildly conflicting testimony over who was the aggressor on the rainy night the 17-year-old was shot while walking through the gated townhouse community where he was staying.
Defense attorneys said the case was classic self-defense, claiming Martin knocked Zimmerman down and was slamming the older man's head against the concrete sidewalk when Zimmerman fired his gun.
Prosecutors called Zimmerman a liar and portrayed him was a "wannabe cop" vigilante who had grown frustrated by break-ins in his neighborhood committed primarily by young black men. Zimmerman assumed Martin was up to no good and took the law into his own hands, prosecutors said.
State Attorney Angela Corey said after the verdict that she believed second-degree murder was the appropriate charge because Zimmerman's mindset "fit the bill of second-degree murder."
"We charged what we believed we could prove," Corey said.
As the verdict drew near, police and city leaders in the Orlando suburb of Sanford and other parts of Florida said they were taking precautions against the possibility of mass protests or unrest in the event of an acquittal.
"There is no party in this case who wants to see any violence," Seminole County Sheriff Don Eslinger said immediately after jurors began deliberating. "We have an expectation upon this announcement that our community will continue to act peacefully."
The verdict came a year and a half after civil rights protesters angrily demanded Zimmerman be prosecuted.
Jurors heard two different portraits of Zimmerman and had to decide whether he was a wannabe cop who took the law into his own hands or a well-meaning neighborhood watch volunteer who shot the unarmed teenager in self-defense because he feared for his life.
The jury deliberated until late Saturday night after Zimmerman's defense wrapped up closing statements and the prosecution concluded rebuttal arguments Friday. Zimmerman, 29, was charged with second-degree murder in the shooting death of Florida teen Trayvon Martin during an altercation last year in a Sanford, Fla. gated community. Zimmerman pleaded not guilty, claiming he shot the teen in self-defense.
The case has drawn intense media spotlight, and a 44-day delay in Zimmerman's arrest after the Feb. 26, 2012 shooting spurred national protests.
The jury panel has been sequestered during the trial with limited contact to the outside world. Jury selection began in the case June 10. Everyone selected to sit on the jury vowed to judge Zimmerman impartially, based only the evidence presented in court, and not on media coverage of the case.
Throughout the trial, prosecutors and defense attorneys painted starkly different pictures of the 29-year-old. Prosecutors portrayed Zimmerman as a vigilante, frustrated by the break-ins in his community that prompted him to launch a neighborhood watch program. They said Zimmerman, who had aspirations to be a police officer, profiled the teen as a criminal and shot him "because he wanted to."
Defense attorneys portrayed Zimmerman as a concerned community member who was "viciously attacked" when Martin "sucker punched" him and began slamming his head into a concrete sidewalk.
In closing statements Thursday, prosecutor Bernie de la Rionda called Zimmerman a "wannabe cop" and said that Zimmerman lied about following the teen through the neighborhood.
"Who started this? Who followed who? Who was minding their own business? Of the two, who was the one that was armed?" de la Rionda asked the jury.
De la Rionda played a non-emergency call placed by Zimmerman the night of the Feb. 26, 2012 altercation, in which he tells a non-emergency dispatcher he is following the teen and uses the terms "f--ing punks" and "---holes." Prosecutors attempted to prove that the language shows Zimmerman acted with ill will or spite, elements of the second-degree murder charge.
In his closing statements Friday, Zimmerman defense attorney Mark O'Mara asked the jury not to "fill in the gaps" in the prosecution's case, arguing that Zimmerman is "factually innocent."
Aiming to prove that Martin wasn't unarmed, as prosecutors said, O'Mara carefully placed a large cement block in front of the panel.
"That's cement. That is a sidewalk. That is not an unarmed teenager with nothing but Skittles trying to get home," O'Mara said. "That's what somebody who used the availability of dangerous items, from his fist to the concrete, to cause great bodily injury...against George Zimmerman. And the suggestion by the state that that's not a weapon that can't hurt someone, that can't cause great bodily injury, is disgusting."
The state's case spanned nearly nine days of testimony from numerous witnesses, including Martin's mother and brother, a medical examiner who conducted Martin's autopsy, a DNA analyst, and numerous neighbors, first responders and police officers who witnessed the fatal altercation or its aftermath.
Key witnesses for the prosecution included Rachel Jeantel, a friend of Martin's who was on the phone with him minutes before he was fatally shot as he was walking to his father's fiancee's home.
Jeantel said she heard Trayvon Martin saying, "Get off, get off" just before his death. But in a lengthy and at times heated cross-examination, defense attorney Don West highlighted changes in her story in an apparent attempt to bring her credibility into question.
During the prosecution's case, jurors saw graphic photos of Trayvon Martin's body and heard from first responders who attempted to save the teen's life. A DNA analyst testified that Martin's DNA wasn't found on Zimmerman's gun, which Zimmerman said the teen reached for.
Also testifying for the prosecution was a medical examiner who said George Zimmerman's injuries following the altercation were "insignificant," and a gym owner who trained Zimmerman who described the 29-year-old as "physically soft."
Trayvon Martin's family members took the stand, telling attorneys they recognized the teen's voice screaming in the background of a crucial 911 call placed by a neighbor the night of the altercation.
When asked who she believed was screaming on the call, Martin's mother Sybrina Fulton said, "Trayvon Benjamin Martin."
The call was played numerous times by both prosecutors and the defense team. Who is screaming has been a key point of contention in the case because it could provide clues as to who was the aggressor in the fatal confrontation.
The defense's case launched late Friday afternoon and spanned nearly three days of testimony. Defense attorneys called to the stand a string of witnesses who knew Zimmerman, playing the tape numerous times in court and asking them to identify the voice. The witnesses, including Zimmerman's mother and brother, said the screaming voice belonged to Zimmerman.
In an emotional moment, Zimmerman's friend John Donnelly said that he was able to identify the screams of men in distress when he was a combat medic in Vietnam.
"There's absolutely no doubt in my mind that is George Zimmerman," Donnelly said, crying. "I wish to God I didn't have that ability to understand that."
Providing crucial testimony for the defense team was Dr. Vincent Di Maio, a nationally renowned gunshot wound expert who said Trayvon Martin was leaning over George Zimmerman when the teen was shot. Testifying for prosecutors, a neighbor, John Good, also said he believed Martin to have been on top in the fatal confrontation.