The former Daily Mail columnist, 48, and the far-right activist, 40, were banned from the app for breaking its rules on hateful conduct in 2020 and 2018 respectively.
This move from Elon Musk, 52, continues his ongoing effort to allow as many figures back onto the platform as possible.
Upon returning to the social media site, Katie expressed her gratitude to the Tesla boss, and promised they would “fight for freedom” together.
She wrote: “Thank you @elonmusk. And thank you to all the Twitter family who have brought Tommy and I back to @X.
“Know this. You are not alone. We are many. And we are stronger together. The fight back for your freedom is on.”
The far-right leader agreed with Katie, and insisted he would use the platform to tell “the truth” about society.
He wrote: “I’ve been censored, attacked, slandered and imprisoned for shining a light on uncomfortable truths that our government wish to hide, the public are now aware I was telling the truth. We have lots to do.”
While some praised Elon for his decision, others have criticised the move as an act of "allowing hateful content".
Nick Lowles, the chief executive of anti-hate campaigners Hope Not Hate, said: “Reinstating Tommy Robinson and Katie Hopkins means Elon Musk is allowing hateful content and misinformation to take place on his watch on X/Twitter.”