The Manchester United footballer marked the occasion with a video message for his fans, which also included a quick version of his famous "Siu" celebration that is usually seen when he scores.
He told his 400 million followers: "Wow, what a number. Now I can say Siu! It's fantastic, what a moment for me. Without you this would not be possible, so from the bottom of my heart I have to say thank you and keep going like that. I will share my life with you, all of the things with you because you deserve it. Thank you very much, let's do it for 100, 200 ... millions."
He accompanied the video with the hashtag #alwaystogether.
His feed has been a mixture of his football exploits with the likes of Manchester United, Juventus, Portugal and Real Madrid, plus showing his family life with partner Georgina Rodriguez and children, as well as modelling shots and paid partnerships.
The sportsman has been active on the social media site since 2012 when he kicked his Instagram journey off with this post:
Ronaldo and family were recently in Dubai for a holiday while there was a break in the Premier League football fixtures.
He was seen sunning himself on the beach at the Bulgari resort, training at the Nad Al Sheba Sports Complex, surprising girlfriend Rodriguez with a special display on Burj Khalifa on her birthday, and meeting Sheikh Mohammed bin Rashid, Vice President and Ruler of Dubai.
He also hung out with thousands of fans at a special event in Expo 2020 Dubai and then caught up with his good friend Sheikh Hamdan bin Mohammed, Crown Prince of Dubai, at the Aura Skypool, the world's highest 360º infinity pool, located on the 50th floor of The Palm Tower.
While Ronaldo is the most-followed person on Instagram, he doesn't have the most-followed account. That belongs to Instagram itself, which has 473 million followers.
The top 10 most-followed people on Instagram:
1. Cristiano Ronaldo — 404 million
2. Kylie Jenner — 312 million
3. Lionel Messi — 308 million
4. Dwayne Johnson (The Rock) — 299 million
5. Selena Gomez — 298 million
6. Ariana Grande — 295 million
7. Kim Kardashian — 287 million
8. Beyonce — 238 million
9. Justin Bieber — 221 million
10. Virat Kohli — 183 million