In the early hours of Thursday morning, the 27-year-old told his five million followers: "Was with my baby mamma tonight."
CLICK ON PHOTOS TO ENLARGE
Rob Kardashian has further fuelled reports he is a father
It comes just four days after Rob posted, and then quickly deleted, another message referring to his child.
"I'm about to go pick up my son from the port and can't wait to watch a great movie with his, suggestions anyone?? He's 6," he wrote.
Speculation that Rob had become a father first began in June, when he wrote a message that read, "And to my baby mama, I loveeeeeee you."
Then in July, the reality star tweeted that he was looking forward to spending Independence Day with his "little man".
Was with my baby mama tonight ,,— ROBERT KARDASHIAN (@robkardashian) September 18, 2014
Rob Kardashian posted this tweet in the early hours of Thursday morning
"Happy I got my son with me for the 4th tomorrow!! Can't wait to watch fireworks with my little man! Hope everyone is safe! X," he wrote.
But hours later, the message had been removed, with Rob claiming it had been a joke.
"Those who know me know I play and I do NOT have a son," he said. "Y'all got my mama emailing me asking if I have a son lol goodness hahaha. GOODnight."
Rob's behaviour on social media has been somewhat erratic of late – in June, he wrote, "no-one will ever understands how much this hurts", before unfollowing everyone except his mum Kris Jenner and sister Kylie.
Rob Kardashian with his mum Kris Jenner
That same month, Kris admitted that her son was "struggling with some stuff".
"Rob is living in LA right now, he is doing well, finding out what makes him tick," she told E! News. "He is struggling with some stuff but my job is to be there for him and love him as hard as I can.
"Everyone goes through stuff and we get through each other's adversity one year at a time."