He broke the previous record held by American actress Jennifer Aniston, reports BBC.
According to the Guinness World Records, the 94-year-old naturalist's follower count raced to seven figures in four hours 44 minutes on Thursday.
His debut post said: "Saving our planet is now a communications challenge."
Last October, Friends star Aniston reached the milestone in five hours and 16 minutes.
In his first video, the veteran broadcaster told followers: "I am making this move and exploring this new way of communication to me because, as we all know, the world is in trouble.
"Continents are on fire. Glaciers are melting. Coral reefs are dying. Fish are disappearing from our oceans. The list goes on and on. Saving our planet is now a communications challenge."
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He joins a varied list of public figures who have at some point held the record for the fastest to gain a million followers.
Instagram record holders:
§ Sir David Attenborough - four hours 44 minutes (September 2020)
§ Jennifer Aniston - five hours 16 minutes (October 2019)
§ Duke and Duchess of Sussex - five hours 45 minutes (April 2019)
§ Kang Daniel, K-Pop star - 11 hours 36 minutes (January 2019)
§ Pope Francis - 12 hours (March 2016)
§ David Beckham - 24 hours (May 2015)
Sir David's total following rose to 2.5 million within 24 hours. However, he is some way behind the most-followed person overall - footballer Cristiano Ronaldo, who has 238 million.