The 62-year-old Singh, who was elected from the Lucknow Lok Sabha constituency, has succeeded Sushilkumar Shinde in the crucial ministry that handles the country's internal security and the task of guarding its international borders with Pakistan, China, Bangladesh, Nepal and Bhutan.
Before assuming charge, Singh paid floral tributes to the country's first Home Minister, Sardar Vallabhbhai Patel, at his statue near Parliament.
He was welcomed with flower bouquets by a number of senior ministry officials led by Union Home Secretary Anil Goswami.
"The contribution of Sardar Patelji to the unity of the country has been tremendous. Country cannot repay what he has done for (it). Sardar Patel was the first Home Minister of the country and I came here to take his blessings before taking charge," he told reporters.
Meanwhile, Kiren Rijiju also took charge as the Minister of State for Home Affairs at his North Block office.
Among the immediate issues that Rajnath, the teacher- turned politician, will have to deal with are tackling Naxal violence, activities of terrorist outfits in the country's hinterland, infiltration from Pakistan, illegal immigration from Bangladesh and strengthening intelligence gathering mechanism.
As Home Minister, two-term BJP president Rajnath will be part of the Prime Minister Narendra Modi-led Cabinet Committee on Security, the country's highest decision-making body on security matters.