Barmaid Georgia Thomson has alleged to The Mirror that she slept with Sam last year, while he was dating the former Hollyoaks star, who is currently competing on CBB.
She provided images of Sam, which she claims were taken in her apartment in Magaluf the morning after their alleged tryst.
But Sam has dismissed her claims.
Georgia told the newspaper Sam informed her about Stephanie that morning: "When he was getting dressed to leave, he said 'I'll see you around', and said 'please don't tweet about it because obviously you know I've got a girlfriend'."
However, a representative for Sam has said: "Sam denies the allegations."
Sam also appeared to allude to the allegations on Twitter when he tweeted: "Don't believe everything you read... It's laughable."
Don't believe everything you read... It's laughable— Sam Reece (@SamReece) January 21, 2016
His relationship with Stephanie has been in the headlines for the past two weeks, as she's been growing very close to housemate Jeremy McConnell, even sharing a kiss with him. Jeremy has since confessed that he's fallen in love with her.
It's led to speculation that Sam wants to end the relationship and is "shocked and heartbroken" but he hasn't publicly spoken about Stephanie since 11 January, when he withdrew support from her on Twitter, but insisted they are still a couple.
"I will be talking to her when she leaves the Big Brother house," he said.
"I would never end a relationship over text or social media. I want to talk to Steph face to face."
On last night's episode, actress Stephanie became upset as she chatted about Sam in the Diary Room, after housemate Gemma Collins brought up how good looking she finds him.
"I've just realised everything," she said. "I've missed Sam so much that I've had to block him out completely. Maybe I've substituted Jez for Sam as I've missed the contact.
"I don't want to look like a tease, it's not like that, I think the world of Jez."
Sam has been inundated with support from CBB viewers who are unhappy at Stephanie's behaviour in the house. So much so, many have pleaded with Sam to enter the house and confront her.
Show host Emma Willis, however, has said this won't happen.
CBB airs tonight, 9pm on Channel 5