William Porter, charged with manslaughter, said he did not call for a medic for Freddie Gray because he did not have a reason to do so.
Gray died after sustaining a spinal injury in the back of a police van during one of its six stops.
He said he checked on Gray during the stops, and he had no signs of injury.
His death sparked protests over police brutality, with the city of Baltimore erupting in rioting, looting and arson on the day of his funeral.
Gray was "unable to give me a reason for a medical emergency," Mr Porter told jurors.
According to the Baltimore Sun, he told jurors that he held Gray in a "life saving position" at the police station for what "felt like an eternity".
"It was a very traumatic thing for me also," he said. "Just seeing him in the neighborhood every day, and calling his name, and not getting a response."
He also faces assault, misconduct in office and reckless endangerment charges, and could receive up to 25 years in prison.
Six other Baltimore police officers will go on trial in Gray's death. All have pleaded not guilty. They will be tried separately and prosecutors hope to use Mr Porter as a witness in the other trials.
Prosecutors have argued that Mr Porter is at least somewhat responsible for Gray's death because he did not buckle him into a seatbelt in the van after he was arrested for running from police, which is department policy, and he did not call for medical attention when Gray needed it.
Mr Porter said Gray asked for help getting off of the floor of the van and denied claims that Gray told him he could not breathe, though he did say he heard him say something about needing an inhaler upon his arrest.
He said in 200 arrests involving the van, he has never belted a prisoner because the wagon is "pretty tight".
Mr Porter told defence lawyers in a pre-trial filing that Gray was always "banging around", referring to a previous arrest in which he allegedly tried to kick windows out of a police car.
Earlier in the trail prosecutor Michael Schatzow said Mr Porter could have saved his life by calling for medical help and it is his duty to keep prisoners safe.
"The defendant alone is on trial for what he did, or more importantly, what he did not do," said Mr Schatzow.
A Baltimore police officer facing trial over a death in custody has testified he did not think the man was hurt until he arrived lifeless at the station.