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.
The 'Vicky Cristina Barcelona' actress delivered her baby at the Ruber Internacional hospital with her 'Skyfall' actor husband, as well as her parents and siblings, Monica and Eduardo Cruz, by her side.
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.
She explained: "From the first second, you feel so much love. It is a revolutionary experience. That's the best way I can describe it. It transforms you completely, in a second. Nature is very wise and gives you nine months to prepare, but in that moment, when you see that face, you are transformed forever."