Usain Bolt and his girlfriend Kasi Bennett.

Usain Bolt has secretly welcomed twins with his girlfriend Kasi Bennett.

The sportsman revealed his babies' unusual names Saint Leo Bolt and Thunder Bolt when he posted the bombshell on Instagram on Sunday.

In a big reveal on Father's Day, Usain shared a heartwarming family portrait including the new arrivals on his Instagram with his legion of loyal fans.

The proud parents posed with their three happy children sat in front of them.

It's the first fans have heard of the news and they got a glimpse of the adorable sleeping children.

Already the couple have been raising their daughter Olympia who was born in June last year.

His 10.1million followers were quick to share their congratulations to the parents in the comments below.

Kasi shared the identical photograph on her Instagram with an emotional message to her lover.

She wrote: "Happy Father’s Day to my forever love! @usainbolt

"You are the rock of this family and the greatest daddy to our little ones. We love you world without end!"

Until Father's Day, the lovebirds had chosen to keep their babies and the pregnancy under wraps.

Usain and Kasi have been romancing for six years and have a beautiful family together.

Last year, the star vowed to be the rock for his family as he started his new chapter with his girlfriend.

He wrote on Instagram: "I want to wish my gf @kasi.b a happy birthday and To let u know I am happy I get to spend ur special day with u.

"I want nothing but happiness for u and will continue to doing my best keeping a smile on ur face. Now we have started a new chapter together with our daughter Olympia Lightning Bolt.

"I look forward to what the future will bring for us but be reassured that I will be the ROCK for this family. I Love you and happy 21th birthday." (sic)

Previously, the runner has said he would advise his children against following in his footsteps into the sporting world.

The sporting champion has made history as the only sprinter to win the 100m and 200m golds at three consecutive Olympics in 2008, 2012 and 2016.

Thoughtfully, the sports star considered it would be a hard feat for his children to follow his achievements.

"That's going to be hard for my kid,” Bolt told reporters at Super Bowl LIV in Miami in 2020.

“If they want to do it, I'm fine with it. But initially I'm going to say don't do it, because I know the pressure that will come along with it."