The 89-year-old actor and the 42-year-old The Only Way Is Essex star were both guests on Channel 4's Late Night Lycett and took part in a Dynasty cat fight spoof sketch after the comedian provided them with a script.
Gemma said: "Shall I do it proper Essex? 'Step aside, Dame Joan! There's a new Collins in town!'"
Dame Joan replied: "I'm the original OG," before Gemma retorted: "You are, honey. I'll give you that but move over b***h because I'm here."
The Dynasty star then lifted her hand and gestured a slap towards the reality star's face, in time with a sound effect.
The GC shared the clip on Instagram, writing: "What an honor to be slapped by @joancollinsdbe I will always be greatful for this moment by such a glamorous dame! [sic]
"Always wanted to be in Dynasty when I was young and I got to meet Joan herself it’s a total moment in my life which is unreal to then be next to her.
"@joelycett what a night !!!"
Dame Joan added on social media: "Great fun and #lorralaughs with @joelycett last night on #joelycettlive with @joeldommett @munroebergdorf and @gemmacollins"
Lycett, 34, said the latest episode of his new Channel 4 chat show had been an "absolutely mad one" and revealed he had had Collins Caterpillar cakes made with both the diva's faces on.
Dame Joan famously starred as villainous Alexis Colby in American soap Dynasty between 1981 and 1989, as well as 1979 film The B***h.
Her more recent TV appearance have included Benidorm, The Royals and American Horror Story: Apocalypse.
The reality star reassured her 2.2 million followers she was "all good" as she explained she had dialled 999 after suffering chest pains.
She later revealed it had been a panic attack.
Watch: Joan Collins says it is 'tremendously rude' to ask a woman her age