Rihanna's shining bright like a diamond by her man A$AP Rocky ... showing her support at his L.A. criminal trial Wednesday.

The singer avoided the media circus outside the downtown L.A. courthouse and came in incognito, showing up inside the courtroom ... she's wearing a black coat and sitting in the front row of the gallery, between Rocky's mom and sister -- with jurors clearly noticing her presence.

Rocky himself walked into the courthouse earlier as cameras flashed in his face and paparazzi shouted questions at him.

We broke the news that Rihanna was gonna pull up at the trial and sit with the regular folks -- and sure enough, she did.

The trial’s just kicking off, and Rihanna’s already in the mix. Last week, TMZ broke the story -- she was a key factor in jury selection, with potential jurors getting grilled on whether they’re fans of hers, since that could sway their judgment.

The couple was seen out and about Tuesday night after the rapper's long day in court ... the famous duo hit the town for dinner at Verse, a swanky L.A. restaurant.

A$AP is on trial for allegedly shooting his former friend A$AP Relli during a heated sidewalk showdown in Hollywood back in 2021.

The D.A. hit him with two felony assaults with a firearm charges after his headline-making 2022 arrest at LAX -- right after he touched down from Barbados.

If convicted, Rocky’s looking at up to 24 years in state prison. This comes after he turned down a plea deal that would’ve had him plead guilty to one charge, with a recommended 180 days in jail and 3 years' probation.