Kim Kardashian says her newborn niece is "absolutely perfect".

The reality star is thrilled after her sister Kourtney gave birth to her second child, a daughter called Penelope Scotland, in Los Angeles over the weekend.

Kim posted a message on her blog to welcome the latest addition to their family and congratulate Kourtney and her partner Scott Disick.

"Welcome to the world, Penelope Scotland Disick!" Kim wrote. "Kourtney gave birth to a beautiful baby daughter on Sunday and the whole family is so thrilled to finally meet her! She is absolutely perfect! I am so excited to be an aunt again and can't wait to spoil little Penelope. I know Mason is going to be the best big brother in the whole world. Congratulations Kourtney and Scott. Xo."

Khlo Kardashian is equally elated at the arrival of Penelope.

The star took to her Twitter account to celebrate the birth of the little girl.

"Such an amazing day!!!!!" Khlo wrote. "Beyond Blessed!"

Ryan Seacrest who produces the family's hit TV show Keeping up with the Kardashians has also sent his well wishes to Kourtney and Scott.

"Congrats proud parents! Love the name (sic)," he tweeted.