He replaces Wang Yang, viewed by many as a leading reformer.
Xinhua state news agency did not say what Mr Wang's next job might be. There is no suggestion he has been sacked.
Mr Hu, 49, a protege of outgoing President Hu Jintao, was promoted to the politburo last month.
Known as "Little Hu", the new Guangdong boss is part of the so-called "sixth generation" of leaders born in the 1960s and lined up to take charge after the next transition of power in 2022.
Mr Hu will be replaced by Wang Jun as party chief in Inner Mongolia, Xinhua said.
The reshuffle follows the confirmation of Xi Jinping as national Communist Party leader last month.
Wang Yang is also a politburo member, but missed out on promotion to its seven-member Standing Committee this year. Analysts say he could become a vice-premier next year.
Mr Wang was considered a rival of the disgraced Bo Xilai, who was sacked this year as Chongqing party secretary.
The party secretary post in the manufacturing hub of Guangdong is among the country's most prominent provincial leadership jobs.
Guangdong, China's most populous province, is often viewed as a testing-ground for reform.
Correspondents say it is not clear what political direction Mr Hu will take.

The graduates were seen driving a convoy of black Mercedes jeeps noisily through Moscow last month, blocking traffic and hanging out of the windows.
Some veteran Russian spies have said such behaviour amounted to treason.
The FSB says those involved will have the conditions of their service changed, without elaborating further.
In a statement (in Russian), it also says some of the FSB Academy's senior officers have already been demoted and some will be sacked.
"The immodest, demonstrative behaviour linked to the short-term rentals from a luxury car company sparked justified indignation among citizens and strong condemnation from the community of military security bodies."
Footage of the celebration was posted online shortly after the incident, attracting hundreds of thousands of viewers.
The FSB is the main successor of the Soviet KGB.
The agency's powers and budget have risen significantly under President Vladimir Putin - himself a former KGB officer.

Umakant Mishra was suspended from his job after being charged with fraud when 57 rupees and 60 paise (92 cents; 56 pence) went missing in his post office.
Mr Mishra told BBC Hindi that  the battle to clear his name reduced him to penury and his family would have starved if not for his relatives.
He is now seeking compensation.
Correspondents say cases in Indian courts can drag on for years, even decades, and official figures show that more than 30 million cases are pending in Indian courts, some dating from 1950.
Mr Mishra's troubles began on 23 July 1984 when he worked in a post-office in the Harjinder Nagar area of Kanpur city in the northern state of Uttar Pradesh.
"I was given 697 rupees and 60 paise ($11.19; Â£6.83) which the post-office had received as money orders and I had to distribute it. I distributed 300 rupees and in the evening deposited the rest with a senior official," Mr Mishra says.
When the refund was checked, it was found to be short by 57 rupees and 60 paise (92 cents; 56 pence).
"I was charged with fraud. I was suspended from my services and a complaint was lodged with the police."
Mr Mishra was jailed briefly and although he was freed on bail shortly afterwards, his long legal battle had started.
"I was summoned 348 times by the court. Initially I had to sell my house in Kanpur, then I had to sell my agricultural land in Hardoi district. I went bankrupt."
He did various odd jobs to support his family and fight the legal battle and says he was helped by his relatives who "helped me and provided me with shelter".
He was absolved of all charges last week after the prosecution could not produce any witnesses in the case.
"I was suspended when I was in my 30s. Now that I have been absolved, I should be compensated. I should get all the money that is due to me," he says.
One of the Chinese Communist Party's rising stars, Hu Chunhua, has been appointed party boss in the powerhouse province of Guangdong.
[NEXT_CONCEPT]
Russia's FSB security service says it will punish a group of new agents who allowed themselves to be photographed lavishly celebrating their graduation.
[NEXT_CONCEPT]
An Indian postal worker who was accused of stealing less than $1 in 1984, has been cleared by a court after nearly 350 court hearings over 29 years.