China's Prime Minister Wen Jiabao has promised $600m (£344m) in aid and loans to Cambodia, much of it earmarked for the construction of dams and bridges.
Cambodian leader Hun Sen has cultivated closer ties with China
Mr Wen agreed the deal with Cambodian leader Hun Sen at the end of his visit to the south-east Asian country.
Half the aid is for a hydro-electric project, with other sums set aside for bridges and new government offices.
The aid deal is one of several recent signs that China is strengthening its ties with Cambodia, analysts say.
In September, Beijing gave six naval patrol vessels to Cambodia, which is a major regional hub for drug and human trafficking.
In recent years, China has also given millions of dollars of aid to the country, struck trade agreements with it and invested heavily in its clothing industry.
Hun Sen has, in turn, described China as Cambodia's "most trustworthy friend".
Cambodian Foreign Minister Hor Namhong told reporters Mr Wen's visit opened "a new chapter in the Cambodian-Chinese relationship".
"After this we believe the Chinese will bring more investment and aid to Cambodia for development," he said.
Among the 11 agreements signed by Mr Wen and Hun Sen are deals securing Chinese aid for restoring the historic Angkor Wat temple and for building a new seat of government in Phnom Penh.
About $200m of the Chinese money has been set aside in a low-interest loan for the construction of bridges spanning the Mekong and Tonle Sap rivers.
Analysts say China is keen to strengthen ties with south-east Asian countries that have sea ports that can serve Beijing's growing hunger for oil from the Gulf.
Cambodia hopes its closer ties to China will help it counter the influence of its rival, neighbouring Vietnam, analysts say.
Hun Sen spent years fighting the Khmer Rouge, Cambodia's Beijing-backed government, but since their overthrow and his rise to power, he has tried to repair relations with China.
Officials associated with the Khmer Rouge, led by the late Pol Pot, are due to go on trial for genocide in Cambodia next year.
They are accused of helping to kill some 1.7 million people in the 1970s - mostly through execution and starvation.