In the latest mega-deal on the booming Avenue of the Americas, Mizuho Americas has signed for 270,000 more square feet at Rockefeller Group’s 1271 Sixth Ave., the former Time-Life Building at West 50th Street, The Post has learned.

That’s on top of a 141,000-square-foot lease Mizuho signed at the iconic address in June — bringing the bank’s total commitment to the 2.1 million-square-foot tower to 411,000 square feet. It’s the redeveloped tower’s largest tenant so far.

MLB originally signed for 400,000 square feet but recently scaled back to 325,000, as The Real Deal reported last week.

Parent firm Japan’s Mizuho Financial Group boasts total worldwide assets of $1.8 trillion. Although Mizuho Americas was expected to add more space at 1271 Sixth, after the June lease, the size of the latest deal came as a surprise.

Earlier reports speculated Mizuho was looking at not more than an additional 200,000 square feet. The firm is relocating employees from 320 Park Ave., where it has trading operations, and other Midtown locations.

Terms were not available. The asking rent for the floors previously leased by Mizuho were in the low $80s per square foot.

Rock Group is in the midst of a $600 million redevelopment of 1271 Sixth designed by architects Pei Cobb Freed & partners — including an entirely new curtain wall.

Mizuho was repped in negotiations by the Savills Studley team of Mitchell Steir, Matthew Barlow, Steve Berliner and David Goldstein. Rockefeller was repped by a CBRE team led by Mary Ann Tighe, Howard Fiddle and John Maher, along with a Rock Group in-house team led by Ed Guiltinan.