Riddle is retiring at the end of May after 14 years in the role.
Before being appointed to his new role, Sadasivan was head of emerging markets equities at Old Mutual Asset Management based in Singapore.
He will be based in London at LGIM and report to Ian King, head of international equities.
"We are delighted to welcome an investor of Suresh’s calibre to LGIM, bringing as he does a wealth of successful fund management experience across a range of markets," said King.
