The 39-year-old actress and her husband Javier Bardem, 44, welcomed their second child into the world in Madrid, Spain, on June 22 and have now registered her name, according to Spain's Vanitas magazine.
Luna means moon in Spanish, while Encinas is Javier's paternal surname and translates as live oak.
Spanish for 'moon': Penelope Cruz and Javier Bardem, pictured here in Madrid in July, have officially named their new baby girl Luna
The couple also have a two-year-old son, Leonardo, together.
Shortly after welcoming Leonardo into her and Javier's lives, Penelope said she felt like motherhood had completely changed her.
New mum: Penelope Cruz reportedly gave birth to her second child with husband Javier Bardem on Monday - the same day that the Duchess of Cambridge welcomed a baby boy into the world
'It transforms you completely, in a second,' the actress continued.
'Nature is very wise and gives you nine months to prepare, but in that moment, when you see that face, you are transformed forever.'
On the same day Prince William and the Duchess of Cambridge announced they'd welcomed a baby boy at St Mary's Hospital in London, weighing in at 8Ibs, 6oz.
Ready to pop: Penelope was last seen on Saturday attending a concert in Madrid despite being heavily pregnant
She recently said of her children: 'I want my son and my kids - if I have more - to grow up in a way that is as anonymous as possible. The fact that his father and I have chosen to do the work we do doesn't give anybody the right to invade our privacy.'
Penelope was last seen just a couple of days prior to her delivery when she looked ready to pop.
Adding to their brood: The pair already have a two-year-old son, Leonardo, who they are notoriously private about
Pregnancy curves: On Saturday Penelope showed off her huge bump ahead of the birth on Monday
Ahead of the gig she was seen walking along with Javier as they headed for a bite to eat while her huge bump hung over her jeans.
The 39-year-old, who was thought to be due to give birth back in June, then looked as if she was ready to welcome her second child at any moment as her stomach swelled against her T-shirt at the Los Veranos de la Villa Music Festival.
Happy families: Penelope and Javier already have a two-year-old son, Leo, who they welcomed in January, 2011