The move ended week-long speculation the government might have to appoint an acting chairman before a final decision.
The 1975-batch Union Territory-cadre IAS officer, who has a doctorate in economics from Boston University, will take over from J S Sarma, who retired this weekend.
Khullar has been given a three-year tenure. In the normal course, he would have retired as commerce secretary next year.
"I will be joining from tomorrow," he told news agency PTI on Sunday.
Khullar will take over as the Trai chief at a time the telecom sector is in crisis.
The Supreme Court has cancelled 122 licences, as a result of which many new players such as S Tel and Etisalat DB have announced they are shutting shop.
The Trai, which was mandated by the court to come out with recommendations on the auction of spectrum, has created a stir by announcing a base price of Rs 3,622 crore (Rs 36.22 billion) per MHz -- 10 times what companies paid earlier.
With GSM players going on the offensive against the regulator's proposals, foreign telcos such as Uninor and MTS have said they would have no option but to quit the country if the recommendations are accepted.
Khullar also has to tackle the contentious modalities and rules for the refarming of 900-MHz spectrum.
He would have to deal with the operators' contention that consumer tariffs would rise substantially due to high spectrum prices.
Khullar has worked closely with many of the top men in power on Sunday. He worked with Prime Minister Manmohan Singh, when the latter was the finance minister, as his private secretary between 1991-93.
He also honed his skills during a five-year spell at the Prime Minister's Office between 1985 and 1990, when his colleagues included current Planning Commission Deputy Chairman Montek Singh Ahluwalia and Principal Secretary in the PMO Pulok Chatterji.
However, Khullar took a break from the government to join the Asian Development Bank in Manila.
He said in a recent interview with this newspaper that the career move was primarily because he had to pay for his daughter's education abroad.
On his return, he joined the Delhi government, where his key achievement after taking over as the commissioner of the sales tax department was to oversee the implementation of the value-added taxation system. In 2004, he was joined the commerce ministry.
Though Manmohan Singh became Prime Minister just a few months later, Khullar was not in the top team in the PMO.
Except a brief stint in the disinvestment ministry, Khullar was virtually in Udyog Bhavan, playing various key roles in the ministry.
The most important was that of a savvy negotiator in the Doha trade negotiations.
He helped dispel the notion that India hampered the negotiations of 2008.
He also played a key role in India signing a series of key free trade agreements, including with Japan, South Korea, Malaysia and the Asean.
The 1953-born Khullar knows four languages: Hindi, English, Punjabi and Malayalam. He was speculated to be the country's next finance secretary after the retirement of Ashok Chawla but was not chosen for the coveted post.
He also turned down an offer from the government to become the Indian ambassador to Brussels.
Khullar's batchmates include Department of Telecommunications Secretary Rentala Chandrashekhar, whose department will have the final say on the auction recommendations made by the regulator before they go to a ministerial group.
Image: Rahul Khullar | Photograph: Press Information Bureau
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