The co-stars (who play loveable couple Leonard and Penny on the sitcom) previously dated for two years, but since their split in 2009 both have remained good friends and gone on to have other relationships.
Recently, Kaley announced she and her ex-husband Ryan Sweeting have divorced and following their split, many were quick to link the US actress to her former flame, Johnny. Hitting out at such rumours, however, both Kaley and Johnny have taken to Instagram to insist their relationship is purely platonic.
Sharing a black and white image of herself and Johnny fist bumping, Kaley told fans: "So sorry to disappoint, but no home wreckers or secret flings going on here. Me and @sanctionedjohnnygalecki are just the best of buds - Leonard and Penny will just have to suffice."
Johnny, meanwhile, made sure to deny such claims himself on his own Instagram page. He posted a similar image of the pair together, adding the caption: "No scandals/ home wreckers/ pregnancies here, y'all. Just profound friendship. @normancook"
Several reports had suggested Kaley had turned to Johnny following the breakdown of her marriage, with sources claiming her feelings for her co-star had never gone away. At the time, a rep for Kaley had slammed such reports as "ridiculous" and "untrue".
Kaley's latest Instagram comes nearly a month after it was revealed she and Ryan had ended their marriage.
Just 21 months after they wed, the former couple decided to go their separate ways. Confirming their split, Kaley released a statement via a spokesperson. It read: "Kaley Cuoco and Ryan Sweeting have mutually decided to end their marriage. They ask for privacy at this time. No further statement will be issued regarding this matter."
Kaley and tennis pro Ryan had enjoyed a whirlwind romance that saw them move in together after their first date. They married in a 'Fire and Ice' themed ceremony in Southern California on New Year's Eve 2013.
Following their nuptials, Kaley had gushed: "Last night I married my sweetheart. Life couldn't be sweeter."
The couple's recent split came as a shock to fans, as earlier this year, Kaley had denied there was any trouble in her relationship.
After she and Ryan were hit by split rumours, Kaley hit back: "I have never been more proud of anyone on this planet. This guy is fighting back from a whirlwind of injuries and not to mention constant 'conversation' regarding our marriage and basically any daily decision we decide to make."
"All of you who take it upon yourselves to trash our marriage, daily workings of our relationship and everything in between, go ahead and keep doing it, cause it only makes us stronger - if you were smart, you would take a look at your own marriage, relationship, job etc, instead of focusing on someone else's."
For exclusive competitions and amazing prizes visit Reveal's Facebook page NOW!