Deal ensures supplies of natural gas
Xi and Berdymukhamedov inspected the gas field, which has proven reserves of 4 to 6 trillion cubic meters of natural gas.
Galkynysh is "another fine example of bilateral energy cooperation for mutual benefits", Xi said.
"The project will inject fresh and strong impetus into energy cooperation between the two countries," Xi said.
Berdymukhamedov said energy cooperation is an important part of the two nations' strategic partnership, which has been driving the development of the Central Asian region.
Turkmenistan is eager to enhance cooperation with China in areas such as natural gas exploitation and transmission, and global energy security, Berdymukhamedov added.
He also said that energy cooperation with China proves that his country's energy export diversification strategy is effective and successful.
China's annual natural gas consumption will reach 327.9 billion cubic meters by 2018, according to ICIS C1 Energy.
Imports are expected to more than triple by then.
"By Sept 3, the total quantity of China's natural gas import contracts were about 119.6 billion cubic meters (excluding the new contract of 25 billion cubic meters with Turkmenistan), which means that there is still a big shortage for China's natural gas imports," said Wang Ruiqi, a senior analyst with ICIS C1 Energy.
She said the new contract with Turkmenistan will ensure China's natural gas supply.
Turkmenistan holds the world's fourth-largest natural gas reserves and possesses vast reserves of oil. It is estimated that 400 million people in China benefit from natural gas from Turkmenistan.
Xi arrived in Turkmenistan on Tuesday. The Central Asia tour is his third international trip since taking office in March.