"I love and support Cory and will stand by him through this," she tells PEOPLE exclusively. "I am grateful and proud he made this decision."
Michele and Monteith took their romance public in April of last year and have been spotted around Los Angeles together, most recently at a Los Angeles Kings game on March 23.
Michele, 26, has been quite vocal about her feelings for her beau, telling PEOPLE in 2012, "He really, really inspires me, and he motivates me and I think he's just so talented."
Michele isn't the only one lending her support to Monteith. "Cory is a beloved member of the Glee family and we fully support his decision to seek treatment," 20th Century Fox TV, which produces the show, told The Hollywood Reporter in a statement. "Everyone at the show wishes him well and looks forward to his return."