John Rabago had previously denied the charges against him and another officer, Reginald Ramones, but Rabago admitted in federal court on Monday that he trapped Sam Ingall in a restroom in January 2018 and gave him an ultimatum: lick a urinal or face arrest.
Ramones pleaded guilty in September.
Rabago initially had claimed his comments were a joke, but Ramones testified that his fellow officer was serious, repeatedly threatening Mr Ingall, who ultimately submitted.
The two Honolulu Police Department officers were arrested in April on a federal indictment charging them with civil rights violations.
Ramones testified that Rabago had pressured officers to lie about the incident and delete text messages between the officers as part of a department cover-up while a civil rights investigation was underway, according to prosecutors.
Court documents revealed that the incident was not the first time that John Rabago had threatened someone in a toilet.
Rabago had also forced another man to dunk his head into a toilet to avoid arrest during a separate incident, according to prosecutors.
On 28 January 2018, several officers responded to a nuisance complaint at restroom area in a Honolulu shopping district. Ramones was among several officers who arrived on the scene.
Ramones propped a bathroom door open while he watched Rabago “in an aggressive tone” threaten Mr Ingall with arrest unless he licked a urinal. Rabago also told Ramones to close the door to avoid his capture on a nearby surveillance camera.
According to reports, Mr Ingall told his sisters that the officers had forced him to sit in urine and shoved his head into the toilet.
Mary Ingall told Hawaii News Now: “He says he wasn’t resisting arrest … They hit him with a stick on his left arm. He has a big bruise. Made him lick the urinal. They also made him sit in pee.”
Mr Ingall didn’t report the incident to police, but one of the officers alerted his superiors, prompting a federal probe, according to reports.
A charge against Rabago for conspiracy to deprive a person of their civil rights carries a maximum penalty of 10 years in prison and a fine of up to $250,000; a charge of deprivation of rights under colour of law charge carries a maximum term of one-year in prison and a fine of up to $100,000.
Rabago’s attorney Megan Kau told reporters: “He’s very remorseful, which is why he took responsibility.”
Rabago was placed on leave from the police department and is expected to be fired.