1/18 9:35 AM -- The minor's attorney, Gloria Allred, tells TMZ Sports ... "It does not surprise me that law enforcement would close their investigation since our client who is a minor and her parents have chosen not to speak to law enforcement to protect their privacy."

Oklahoma City Thunder guard Josh Giddey will not face charges stemming from a police investigation into whether he had an improper relationship with an underage girl ... TMZ Sports has learned.

Newport Beach PD tells us the case -- which was launched back in November -- is officially closed ... after officials found no corroborating evidence.

In other words, no one involved was willing to talk ... and 21-year-old Giddey is in the clear.

As we previously reported, Giddey was the center of serious allegations made on social media during Thanksgiving weekend ... with photos and videos showing him interacting with a girl believed to be a high school junior at the time.

The NBA looked into the claims at the time ... but once Newport PD got involved, Adam Silver said it would take a "backseat" and let law enforcement do its thing.

Our sources told us at the time that the minor and her family were unwilling to cooperate with investigators ... which was a major roadblock in the process. We were also told the minor even lawyered up with prominent attorney Gloria Allred.

Giddey -- who has not publicly addressed the allegations -- has continued to play for the Thunder throughout the probe ... and has faced jeers from opposing fans on numerous occasions.