WASHINGTON — It didn’t take long for Knicks rookie RJ Barrett to get redemption from his college career that ended too early — in the Elite Eight at Capital One Arena in March.

With five seconds to go in the same venue on Monday night, the 6-foot-7 shooting guard drained two game-sealing free throws with the club up two points to put a sweet capper on his Knicks debut — a 104-99 win over the Wizards.

Barrett’s stirring second-half performance saw him score 14 of his 17 points after intermission following a slow start.

“Oh yeah,” Barrett said when asked if he was thinking of Duke’s NCAA Tournament loss to Michigan State. “The same arena I missed a free throw that cost us the NCAAs. I got to redeem myself.”

During March Madness, Barrett missed a potential game-tying free throw with six seconds left against the Spartans in the Elite Eight. Then his first half Monday in Washington was wretched. Barrett was 1-of-5 overall and 0-for-4 from behind the arc for three points at halftime.

He finished with 17 points on 6-of-13 shooting, making both of his 3s in the second half. He grabbed seven rebounds with three assists. Barrett also hounded Washington shooting guard Bradley Beal into a poor shooting outing. Knicks coach David Fizdale called Barrett “fearless” after the win.

“Everybody on the staff told me to keep going, keep pushing,” Barrett said. “It was great to have that support. I had to figure the game out.”

Whether Fizdale wanted to draft Barrett at No. 3 in the NBA draft or not, the Knicks coach is going to give him every opportunity to succeed. Barrett played a whopping 39:20 in his Knicks debut. Fizdale’s willingness to roll all night with Barrett paid off.

The 19-year-old looked like the best player on the floor in the second half and was the key reason the Knicks survived after Marcus Morris’ senseless ejection 14 seconds into the third quarter.

“That’s a player,” Fizdale said. “When you got a guy who really has belief in himself and doesn’t lose confidence or waver, he’s always engaged in the game. He always feels he has a chance to get it going.”

Barrett will probably burn out if 39 minutes is the norm, but Fizdale wanted to get Barrett’s confidence up. The casuality was second-round pick Ignas Brazdeikis, who became a DNP. No other Knicks player saw more than 29 minutes.

“He’s 19,” Fizdale said of Barrett. “And he’s got to play. It’s the same thing I did with Kevin (Knox). These guys have to play, see situations and feel out the game. At the same time I wanted to have him out there because he was playing so well.

“You can see when the lights are on, this kid has an edge to him. He just took over the game for a stretch.”

Barrett loved the excess minutes. For now, the Knicks are minus shooting guards Damyean Dotson (shoulder rehab) and Reggie Bullock (spinal fusion surgery).

“However long coach wants me to play, how short coach wants me to play, I’m cool,” Barrett said. “It was fun for me. Who wouldn’t want to (play 39 minutes)?”

Barrett looked excellent in the open court and is always looking to attack the basket with his size and strength. He was most successful with Frank Ntilikina at point guard. Barrett’s 3-point shooting is a big issue, but the southpaw is crafty enough to find a way to rack up numbers and does other things.

“I did the same all game,” Barrett said. “I played defense and let the game come to me. My teammates were finding me and kept finding me.”