The Argentina worldwide, who already wears the armband for his country, takes over from Iniesta following his move to Vissel Kobe at the end of the 2017-18 campaign.
The Argentina captain takes the club's armband from Iniesta, who succeeded Xavi Hernandez as captain in 2015 but left for Japanese club Vissel Kobe in June.
The second captain will be Sergio Busquets.
The Spain midfielder appeared to symbolically hand over the captain's armband to Messi towards the end of his final Clasico appearance against Real Madrid in May.
Sergio Busquets takes over as vice-captain while Gerard Pique and Sergi Roberto have joined the four-man leadership team, with the quartet all having come through La Masia before graduating to the first-team.
Messi, who has been at Barcelona since the age of 13, is one of four players to be given new responsibilities by the Catalan club.
Messi remains Barcelona's all-time top scorer and the recipient of no less than five Ballon d'Or awards during his time at the club.
Roberto's elevation is in spite of his only being a first team regular for three seasons.
Roberto takes over as the fourth captain, filling the gap left by Messi's worldwide team-mate Javier Mascherano following his move to Hebei China Fortune in January.
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