The exit of Glocksen, 22, of Naperville, Ill., trimmed the number of finalists to eight. The winner will be chosen in May.
Judge Randy Jackson had said on Tuesday's program that Glocksen — the show's resident rocker girl, with her red highlights and all-black outfits — gave a "nice, controlled performance" of the classic song "Smile."
His colleague Simon Cowell, who has griped that Glocksen was not in the same league as other finalists, sported a frown.
Glocksen let out a sob when she heard the result but managed to sing through her tears while performing on "Idol" for the last time.
Haley Scarnato and Phil Stacey had the next-lowest vote tallies in the phone balloting, which drew about 33 million calls and text messages.
Scarnato, 24, of San Antonio, is not opposed to showing a little skin on "Idol." Though Cowell derided her take on "Ain't Misbehavin' " as a bit "pageant-y," he applauded her "nice legs."
Cowell said Stacey's performance of "Night and Day" was too dark and dreary, while Tony Bennett — this week's venerable guest coach — heaped praise on the 29-year-old Floridian, calling him one of the best singers he's heard.
Wednesday's program also featured a performance by Michael Buble, who sang "Call Me Irresponsible."
Among this week's survivors were favorites Melinda Doolittle, LaKisha Jones and Jordin Sparks. Sanjaya Malakar — the stringbean teen who's anything but a judges' favorite but still manages to rack up viewer votes — also made it through.
In its sixth season, "American Idol" continues to rule the ratings. It has attracted 26 million to 37 million viewers per telecast this season.