Italian Antonelli, who has won one sprint and one feature race for Prema Racing in Formula Two this year, will join George Russell at Mercedes, with Hamilton switching to Ferrari.
Antonelli said the move up was a dream come true and thanked the Mercedes team for supporting him and having faith in him.
"Reaching F1 is a dream I've had since I was a small boy... I am still learning a lot, but I feel ready for the opportunity," he said in a statement.
"I'm also really excited to become George's team-mate. He came through the team's junior programme just like myself and is someone I have a huge amount of respect for ... I am looking forward to learning from him and working together to deliver on track."
Russell said he was excited to partner with Antonelli.
"I'm really excited to be partnering with Kimi for 2025. His record in junior formula has been formidable and his promotion is truly deserved," he said.
"I know how much of a support Lewis was for me throughout my time as a junior driver and since I've been his team-mate. I've learned so much from him and I hope to play a similar role for Kimi."
Mercedes F1 boss Toto Wolff said Russell and Antonelli were the perfect line-up for the team to begin its next chapter.
"Kimi has consistently shown the talent and speed needed to compete at the very top of our sport," he said.
"We know it will be another big step up, but he has impressed us in his F1 testing this year ... in George, he has an experienced team-mate from (whom) he can learn and hone his craft."
Russell is eighth in the standings ahead of the Italian Grand Prix in Monza, while Hamilton is sixth.
