A four-year-old Texas boy born without arms or legs yells' "I'm walking", as he takes his first steps in an emotional video posted by his mother.

The toddler from Denton, a city in the north of the state, is seen in the footage inching upright across the floor of his home into the arms of Cole Greene, the partner of his mother Katie Whiddon.

Whiddon said it was the first time her son, Camden, has been able to stand and move on his own.

"With some coaching and a pep talk from his Daddy, CAMDEN FINALLY LEARNED HOW TO WALK!," she wrote in her Facebook post last month.

"I cried like a baby! I've been looking forward to this moment for the past 4 years and it finally happened! I am SO proud of Camden and I am SO BLESSED to be his Momma!"

In the video, which has now gone viral, Greene says to the toddler: "You're sitting up all by yourself. You're not falling."

Camden replies: "I'm coming to you. I'm walking."

"You're walking buddy," Greene confirms.

The video also shows Camden's little sister, Ryleigh, three, looking on as her big brother gets close to Greene. As soon as Camden falls into his arms, she jumps up and down in celebration.

Whiddon was 18 weeks pregnant when doctors read her latest ultrasound scan which alerted them that her son Camden would not have any arms or legs when he was born.

Camden was diagnosed with amelia-phocomelia, a birth defect that leaves a baby without one or more of its limbs.

"When I first found out he wouldn't have arms or legs, I thought that he would just be a vegetable," Whiddon told People magazine.

"I didn't know what to expect. I had just turned 19 and I didn't know, 'Am I gonna be able to be good enough for him?'"

But she added: "When he was around two months old, I had him laying on my bed. I saw him out the corner of my eye and he was hitting the toys with his arms. I started to cry, I was just so happy and that confirmed that he was gonna be able to do things. I had so many doubts at first, but he's just shown me how I don't need to doubt him."