Jalen Brunson traded his Knicks Jersey for a detective’s badge on Thursday night’s “Law & Order: Special Victims Unit” … and the NBA champ handed his real-life pal Mariska Hargitay the clue that blew the whole case open.

Turns out the king of New York is also its best defender … of justice.

JB played Detective Jay Madison in the episode titled “Forsaken.” His scene hits about 20 minutes in and runs roughly a minute … with Brunson posted up at a pizzeria counter, slice in hand, when Hargitay’s Captain Olivia Benson walks in with questions.

Watch the start of the scene closely, too … JB’s dad, Rick Brunson, gets a cameo by strolling into the joint right before Benson. Rick’s Knicks roots run as deep as his son’s, having played on the 1999 team that won the East and joining the coaching staff as an assistant in 2022. He even won a ring alongside his son.

Back to the show, before getting down to business, Benson congratulates Madison on his recent promotion … and his answer is a sly little nod to the Knicks’ title run: “Thanks, Captain. Focused on my goals. Gave 110% and things worked out.”

Benson’s team is investigating the sexual assault of a teenage girl at a family wedding. Madison fills her in on a traffic stop where he’d pulled over the girl’s stepdad and stepbrother … and found a metal self-defense keychain on the son. As for why he didn’t take it? “Legal gray area.”

Turns out, that detail was the break Benson needed. It pointed her toward the stepbrother and led to his arrest.

Brunson kept his trademark poker face the whole time … and he didn’t come back for the rest of the episode.

The Knicks star’s review of the gig? “Really fun. A lot of fun.”

It’s Brunson’s first scripted drama role, but not his first rodeo on NBC. He hosted the “Saturday Night Live” Season 52 premiere in September, a few months after leading the Knicks to their first title in 53 years.

As for Hargitay, she’s a regular at Madison Square Garden games and even rode on the Knicks’ championship parade float. The two also teamed up for a prerecorded bit to open the Emmys she hosted two months ago.