Ariana Grande's 'Victorious' Costars Support Her on 'Eternal Sunshine' Tour

Ariana Grande's concert on her "Eternal Sunshine" tour is bringing out tons of winners ... because a couple of "Victorious" cast members came out to show their love for the star.

The pop star rocked the Kia Forum in Inglewood, California Wednesday night ... and standing in the VIP section were a pair of her former costars on the hit Nickelodeon show ... Elizabeth Gillies and Matt Bennett.

Check out the clip ... the Hollywood Arts High alumni are rocking out to one of Ariana's tracks ... belting out the lyrics along with their longtime pal.

Ariana, Elizabeth and Matt have known each other for nearly 20 years ... having met while shooting the first season of the show in late 2009.

All three cast members are credited for each of the show's 57 episodes ... and Matt even played a love interest to Ariana's character of sorts ... with the pair sharing a couple of onscreen smooches.

This is just the first of Ariana's three shows in Inglewood -- she returns to the stage Friday and Saturday night -- plenty of time for Victoria Justice and other cast members to reunite with the star before she leaves for Austin, Texas.