Governor E.S.L. Narasimhan administered him oath of office and secrecy at a public ceremony held at the Indira Gandhi Municipal Stadium at Vijayawada.
Reddy took oath in Telugu, swearing in the name of God, though He was interrupted several times as the cheerful crowd kept chanting his name.
In Andhra Pradesh, where YSR Congress decimated the Chandrababu Naidu-led TDP in sweeping victory by bagging 49.9 per cent votes in the assembly election.
A day ahead of oath-taking, Reddy undertook a thanksgiving pilgrimage to various places of worship including the Tirumala Venkateshwara Temple, Kanaka Durga Temple, Kadapa Dargah and Pulivendala CSI Church.
Born on December 21, 1972, Reddy is the son of former Andhra Pradesh Chief Minister YS Rajasekhara Reddy. He was earlier a Member of Parliament from Kadapa Lok Sabha constituency.
The YSRCP got an absolute majority in the 175-member Assembly, ousting N Chandrababu Naidu-led Telugu Desam Party (TDP) from power in the state. The party won 151 seats, whereas TDP's strength was drastically reduced from 102 in the 2014 Assembly elections to a mere 23.