The 51-year-old long distance swimmer set off from Japan's east coast on Tuesday in a bid to become the first person to swim across the world's largest expanse of water.
Lecomte plans to reach San Francisco in six months in an attempt to raise awareness about plastic contamination and other kinds of oceanic pollution.
He said that he plans to swim for eight hours daily. He will be accompanied throughout the journey by a support boat, where he will rest, eat and sleep before restarting his swim every morning.
"More than six years of preparations have lead to this moment. Finally ready to start my swim across the Pacific Ocean," Lecomte, who became the first person to swim across the Atlantic in 1998 – in just 73 days – wrote on Twitter.
The French swimmer's two children, Max and Ana, swam out with their father for a few hundred meters before leaving him to continue his quest.
"I am very anxious to start right now," Lecomte said as he prepared to take to the waves.
"I am not an Olympic swimmer, but I am an adventurer in the way that I push my limits," he added.
After successfully completing the Atlantic swim in 1998, Lecomte vowed "never again" to set off on a long-distance sea journey.
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More than 27 organizations, including some medical and oceanographic bodies, will be benefiting from Lecomte's Pacific odyssey. The French swimmer's support team will be gathering data about ocean pollution during the expedition throughout the journey.
Lecomte, whose Pacific swim began on World Environment Day, said he was concerned about the harmful effect of plastic pollution on marine ecosystems.
"I want as many people as possible to understand that each one of us has to take action, because the ocean is in peril right now, and if we don't change anything, it's going to be even worse in a few years," Lecomte said in an interview with GQ magazine published on Monday.
Part of Lecomte's journey includes the Texas-sized garbage pile that floats between Hawaii and California.
shs/rc (AFP, dpa, Reuters)