KATHERINE HEIGL and her husband JOSH KELLEY are par­ents again!

The couple secretly adopted a second child, an 8-day-old baby girl from Louisiana, in early April, The ENQUIRER has learned.

“The whole adop­tion process for Katherine and Josh’s second child was like a covert operation,” re­vealed a source.

“As soon as they got the word the baby was born, Katherine and Josh flew from California to Loui­siana to pick up their new daughter.

“But they didn’t want to let the cat out of the bag too soon about their new little girl.”

Katherine wanted the infant to have a chance to get acclimated to her new family and surroundings before she introduced her to the world.

“To keep the whole thing under wraps, a friend sat with their baby five rows in back of them on the plane trip back to Los Angeles,” said the source.

“Only a select few people even know their newborn baby’s name!”

The former “Grey’s Anatomy” star and her musician hub­by already have a 3-year-old daughter, Naleigh, whom they adopted from South Korea in 2009.

Despite the secrecy sur­rounding their latest adop­tion, proud dad Josh has had a tough time keep­ing his excitement in check!

Accord­ing to the source, Josh spilled the beans to one pal, proclaim­ing: “I’m a dad again! Can you believe it? I’m so happy! I know there will be a lot of sleepless nights coming up for me and Katherine, but I don’t care. I’m tickled pink!”