Kylie Jenner celebrated Tyga's birthday with a racy picture of them together.

Kylie Jenner and Tyga [Instagram]

Kylie Jenner and Tyga [Instagram]

The 19-year-old reality TV star took to Instagram to share a topless picture of herself as she straddled the 27-year-old rapper and she captioned it, "happy birthday baby".

She added another picture of them both in a similar pose and wrote: "Irreplaceable."

Last week Kylie and Tyga's son King Cairo, four, threw the rapper an early surprise birthday party and she shared a number of videos on Instagram.

One video showed King Cairo standing on the kitchen table surrounded by balloons, whilst the lip kit mogul held onto his waist for safety. As Tyga walked in the door, they started to sing him 'Happy Birthday', leaving the 'Rack City' hitmaker completely stunned.

Another featured Kylie handing Tyga a painted ornament of the word "dad", made by the youngster.

Tyga said to his son in the video: "Aw thank you man! Gimme a high five, gimme a high five!"

King Cairo instead went in for a cuddle, before being picked up by his dad.

And Kylie spared no expense on the wide spread of food that was laid out for the 'Ayo' rapper, making sure to include some his favourite items such as fried chicken, and two separate cakes.

The rest of the night was spent playing around with Tyga's son - whom he shares from his previous relationship to Blac Chyna, who is now engaged to Kylie's half-brother Rob Kardashian - as one video captured the boy in a fabric tunnel toy, giggling and screaming as his dad went to catch him.

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