Ansari, 75, was administered the oath by the President at a grand ceremony at Rashtrapati Bhawan.
He had defeated NDA candidate Jaswant Singh with a comfortable margin earlier this week for his re-election as Vice President.
Ansari had polled 490 first preference votes out of 728 valid votes cast in the election held on August. Jaswant Singh polled 238 votes.
Ansari got more votes than he had secured in the 2007 election. He had the backing of the Congress and its allies in the UPA, including the Nationalist Congress Party, DMK and the National Conference.
After Sarvepalli Radhakrishnan, Ansari is the only person to have been re-elected to the office.
