The couple quietly wed Monday, says her rep, Brad Cafarelli.
According to a source, they were married in the Beverly Hills courthouse's ceremony room under an arch of green silk foliage and white flowers. They were casually dressed, with Alba wearing a long blue dress and her hair back in a ponytail.
"She looked happy but nervous," the source says.
Warren, in a white shirt and brown pants, arrived with Alba at about 11:30 a.m. on Monday, applied for a marriage license and waited about 40 minutes for the paperwork to be processed before a staff member from the courthouse married them, the source says.
Nobody else attended the wedding, the source adds.
Alba, 27, is expecting a daughter with Warren, 31, this summer. The couple met while filming Fantastic Four and were engaged last December.
After confirming her pregnancy, Alba said she's been in "frantic nesting mode," taking time off from acting to get the house ready for the baby.
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Additional reporting by ELAINE ARADILLAS