Bynes was not in court Friday to hear the decision because doctors at the psychiatric hospital where she has been staying the last few weeks requested and were granted a month-long extension of her involuntary psychiatric hold, the celebrity news website said.
Bynes' lawyer spoke on her behalf at Friday's hearing, saying the actress opposes the conservatorship and doesn't believe it's necessary.
The temporary conservatorship is to last until Sept. 30, at which time the judge will hold a formal hearing on whether to make it permanent, TMZ said.
Bynes, 27, was placed on an involuntary psychiatric hold in late July after she allegedly caused a disturbance when she tried to break into a retirement community in her hometown of Thousand Oaks, Calif.
The following night, she reportedly set a small fire in a driveway, which led to police involvement and her eventual hospitalization.
The former "What I Like About You" star's parents, Rick and Lynn, are seeking conservatorship of Bynes because they feel she is a danger to herself and others.
The actress is known for her work on TV's "What I Like About You" and "The Amanda Show," as well as in the films "Hairspray" and "What a Girl Wants." Her most recent major screen credit was a supporting role in 2010's "Easy A."
Her once-bright career has been eclipsed in recent years by her bizarre Twitter rants and frequent brushes with the law.