Reality TV star Kylie Jenner has sparked rumours that she's engaged to her boyfriend and rapper Tyga after she shared a photograph of a huge sparkler on her ring finger.

The 18-year-old shared the photograph on Instagram, reports aceshowbiz.com.

Kylie wore the new sparkler on her right hand while she held onto the steering wheel of her car.

She captioned the image with only a Santa emoji.

While she didn't address the engagement rumour, Kylie posted a second image of the diamond ring on her Twitter later on Thursday.

"Merry Christmas," she wrote alongside a heart emoji.