The reshuffle comes when Singh's coalition government has been hit by a number of scandals and corruption allegations, and accused by the opposition of policy paralysis at a time of slowing economic growth and high inflation.
After the Cabinet was sworn in, Singh told reporters he was confident the new ministers would be able meet the challenges awaiting them, the Press Trust of India news agency reported.
"It is a combination of youth, experience and relevance to the portfolios that have been entrusted to the ministers," the prime minister said.
Among the changes, Law Minister Salman Khurshid will take over as minister of external affairs, succeeding from S.M. Krishna, 80, who resigned to make way for younger ministers.
Khurshid will be one of the "big four" ministers, along with the heads of the Home, Defense and Finance ministries, who retained their posts.
Ashwani Kumar, a Supreme Court lawyer who is close to Singh, will succeed Khurshid in the law ministry with Cabinet rank.
The one major surprise in the reshuffle was that Rahul Gandhi, the scion of the Nehru-Gandhi family and tipped to become the country's next leader, did not joint the new Cabinet but decided to work for his Congress party.
The Times of India said the new Cabinet, however, brought in several fresh faces in keeping with Rahul Gandhi's wishes. The new set up includes 17 newcomers, including Jyotiraditya Scindia, Sachin Pilot, Manish Tewari and Jitendra Singh, who have all been given independent charge of ministries, while Ajay Maken, 48, formerly sports minister, was elevated to Cabinet rank.