Senior bureaucrat Rajesh Kumar Singh took over as defence secretary on Friday.
The 1989-batch IAS officer from the Kerala cadre assumed charge at the South Block, where the ministry is housed.
Before taking charge, Singh laid a wreath and paid homage to the fallen heroes at the National War Memorial.
He had assumed charge as officer on special duty (defence secretary-designate) on August 20.
"The nation will remain forever indebted to our brave soldiers who make the supreme sacrifice in the service of the motherland. Their extraordinary bravery and sacrifice is a source of strength and inspiration for us to make India a safe and prosperous nation," he said.
Singh held the post of secretary in the Department for Promotion of Industry and Internal Trade under the Ministry of Commerce and Industry from April 24, 2023, to August 20, 2024.
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Prior to that, he was secretary in the Department of Animal Husbandry & Dairying under the Ministry of Fisheries, Animal Husbandry & Dairying.
Singh has held many other important positions in the Union government, including director of works and urban transport in the Ministry of Urban Development; commissioner (lands) in the Delhi Development Authority; joint secretary in the Ministry of Petroleum and Natural Gas; joint secretary in the Department of Agriculture, Cooperation & Farmers Welfare; and chief vigilance officer in the Food Corporation of India, the ministry said in a statement.
He has also held charge as urban development secretary and finance secretary in the Kerala government.
Singh succeeds Giridhar Aramane, a 1988-batch IAS officer of the Andhra Pradesh cadre, who superannuated on Thursday.