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.
One of the Chinese Communist Party's rising stars, Hu Chunhua, has been appointed party boss in the powerhouse province of Guangdong.