Clinton met with Prime Minister Yingluck Shinawatra as flooding continued to plague areas around Bangkok, the capital.
Thai authorities announced, however, that water in the capital is receding and all main streets will be dry in two weeks, providing good news after months of floods that have killed 564 people nationwide.
Some 20 of Thailand's 77 provinces have been hit by floods since late July, mostly in northern and central areas, and more than one-fifth of the country's 64 million people have been affected.
The flooding has scared away tens of thousands of tourists.
"During the past century we have stood by each other in times of challenge and we are proud to stand by you now in this time of challenge, as you contend with the worst floods in your nation's history," she said in a news conference with the Thai leader.
She said the U.S. was providing both military and civil assistance "to save and restore lives and to support Thailand's long-term rebuilding and recovery," and that teams were currently assessing how best to help.
The U.S. is already providing medical assistance and the U.S. Navy ship Lassen is in a Thai port with crew and helicopters to help relief efforts, Clinton said.
She said the U.S. would help reopen Bangkok's inundated Don Muang domestic airport and rehabilitate flooded police stations.
Washington is also consulting with the Thai government on how to restore important cultural sites, such as the ancient capital of Ayutthaya, which is in one of the most badly hit areas.
Clinton is to visit a flood victims evacuation center on Thursday. Also in town was U.N. Secretary-General Ban Ki-moon, who visited some flood-affected areas Wednesday.
Many areas remain flooded, especially those to the west and east of Bangkok, and it is still expected to take weeks for all that water to reach the Gulf of Thailand. The runoff spread to some sections of Rama II, a major road in Bangkok, but vehicles were still able to drive through, officials said.
But the government appears to have averted a worst-case scenario in which the densely populated and economically critical center of Bangkok would have succumbed.
The Bangkok Metropolitan Authority said the overall situation in the capital is improving quickly, especially in Don Muang, where the domestic airport is located, and Lad Phrao, a district studded with office towers, condominiums and a popular shopping mall.
Lad Phrao intersection is expected to be totally dry by this weekend, and all other main streets will be back to normal within two weeks, Bangkok Gov. Sukhumbhand Paribatra said.
Many Bangkok neighborhoods have been under knee-deep, and even waist-high, water for weeks.
But government efforts to pump the water into the Chao Phraya river seem to be paying off, according to the latest reports.