Van Dyke, who is charged with first-degree murder for shooting Laquan McDonald 16 times in 2014, testified for about 90 minutes.
He recounted the day of the shooting, leading up to his 9 p.m. shift at the South Side police district and how he and his partner, Office Joe Walsh, responded to a distress call near West 40th and Pulaski Road.
Van Dyke described driving up to the scene and seeing McDonald.
WATCH: Jason Van Dyke testifies in his defense, Part 1
"His face had no expression, his eyes were just bugging out of his head. He had these huge white eyes staring right through me," Van Dyke testified.
He appeared to get emotional as he told the court that McDonald turned his torso toward him and waved the knife toward him. Van Dyke testified that he continued shooting because he saw McDonald's shoulder rise and that he still had the knife in his right hand.
"I just kept looking at the knife and I shot at it. I just wanted him to get rid of that knife," Van Dyke said.
LIVE BLOG: Jason Van Dyke trial on Laquan McDonald murder charges
In cross-examination the prosecutor picked apart Van Dyke's narrative, using video and animation from the defense which she said does not show McDonald waved the knife or started to get up from the ground after being shot.
At times Van Dyke seemed to contradict his own account from the night of the shooting.
Prosecutors cross-examining Van Dyke ask him if he reported, after the shooting, that McDonald raised the knife over his shoulder. Van Dyke said: "I don't remember exactly what I said immediately after the shooting. I was still in shock."
WATCH: Jason Van Dyke testifies in his defense, Part 2
Van Dyke's testimony could impact the rest of his life.
"It was extremely important for the jurors to see not the man who fired the 16 shots into Laquan McDonald, but the man himself," said Bob Milan, former prosecutor and ABC7 Legal Analyst.
Tuesday morning, defense attorneys called a psychologist Dr. Lawrence Miller to the witness stand to testify about the mindset of an officer.
Dr. Miller is a psychologist specializing in psychological effects of stress on police officers. He has consulted on several cases of police use of force and the things that officers react to during a stressful situation.
"In an emergency situation, if something is bad, you gotta do something," Dr. Miller said. "You've got to escape. But wait, you can't just escape, you have to neutralize the threat because that's your job and that's essentially what explains what we're seeing on the slide."
He also testified that in times of stress, like the use of deadly force, officers can sometimes have tunnel vision or even tunnel hearing.
WATCH: Jason Van Dyke testifies in his defense, Part 3
The defense is expected to finish presenting their case Wednesday, at which time the prosecution will call rebuttal witnesses.
WATCH: After the Gavel: ABC7's Leah Hopes insights from inside the courtroom
Freelance reporter Zachary Siegel was taken into custody after the judge said he was recording the proceedings, in violation of the judge's decorum order. The judge ordered him held in lieu of $1,000 bail. He needs to post $100 to be released. A hearing was scheduled for Oct. 31. Other journalists pooled their money for the freelance journalist. He is expected to be released at the end of the day.
WATCH: Reporter taken into custody for recording during trial
Organizers are planning for a massive boycott if Van Dyke is acquitted of murder. They say they would want to shut down Chicago for a day, including possibly an expressway.
Laquan McDonald Shooting, Jason Van Dyke Case Timeline
"Nobody go to school, nobody go to work, nobody drive the buses, nobody go to work at the hotels, nobody go to work anywhere and no shopping" said Father Michael Pfleger.