Southeast Asia is generally open to US citizens, with different degrees of visa-free access throughout the region.
Most countries in Southeast Asia allow fairly easy visa-free entry, or visa upon arrival, for stays ranging from two weeks to three months. The only exception is Vietnam, which requires US passport holders to get prior visa approval at a Vietnamese Embassy or Consulate.
So if you're planning to hop from country to country in Southeast Asia, be sure you're prepared long beforehand with the proper requirements, so you don't hit a wall at the immigration gate.
Visa Requirements for Brunei
US passport holders are not required to obtain a visa to visit Brunei; the Entry Pass permits a maximum stay of 90 days. For more information on entry into Brunei, proceed to the Brunei embassy's official site.
Visa Requirements for Cambodia
US passport holders may obtain a visa on arrival; just provide $30, a filled-out visa application form, and a passport-size photo. Your visa will let you stay on for a maximum of one month. Cambodia also offers a slightly more expensive, but vastly more convenient, Cambodia e-Visa that you can apply for online, for an additional $7 fee.
To apply for a Cambodia visa while in the U.S., proceed to the Embassy of Cambodia's official site.
For more information on getting a Cambodia visa, read this article: Cambodia Travel Information.
Visa Requirements for Indonesia
As of March 2016, the Indonesian government allows citizens of 169 countries to enter visa-free. US passport holders, in particular, may get one of these, but the fee depends on how long you plan to stay (30-day maximum).
No extensions are allowed on visa-exempt entries, and no adjustment to a different visa status will be entertained. More information in the official Indonesian Directorate-General of Immigration website.
Alternatively, you can get a visa on arrival for US$ 35 upon arrival at the airport, and extend it another 30 days for US$30. More information in the official site.
To apply for an Indonesian visa while in the U.S., proceed to the Embassy of Indonesia's site for consular services to get started. The embassy will not accept cash for visa fees; only company checks, cashier checks, and money orders will be honored. For more information, read this article: Indonesia Travel Information.
Visa Requirements for Laos
U.S. visitors to Laos can get a visa on arrival, or obtain a visa in advance either by visiting their nearest Laos embassy, or applying for a Laos E-Visa.
Visa on arrival for US passport holders may be purchased for $30; maximum 30 days' stay. Prepare two 2x2 passport photos (or an extra feefor them to scan your passport photo) and fill up a visa form from the visa booth.
The Laos eVisa removes the need to travel to an embassy, or generate paperwork, for a one-time payment of US$45 (U.S. citizens). Just visit the official eVisa site, fill up the online application form and upload documents, pay via a secure portal, and wait for the approval letter to be emailed to you.
Present a printout of the letter at the eVisa line at either the Wattay International Airport in Vientiane or the border crossing at the Lao-Thai Friendship Bridge crossing into Vientiane.
For more information on getting a Laos visa, read this article about Laos visa requirements.
Visa Requirements for Malaysia
US passport holders visiting Malaysia need not get a visa if visiting for social, business or academic purposes. Visitors can stay, visa-free, for a maximum of 90 days. Read the official Malaysian Embassy site for more information.
Visa Requirements for Myanmar
To enter Myanmar, you need a valid passport with at least six months' remaining validity and a valid visa for Myanmar. A “visa on arrival” scheme is open only for tourists from Southeast Asia, China, India and a few other countries in Asia and Europe—American tourists are not included.
Fortunately for U.S. travelers, Myanmar offers an “e-Visa” - just log onto their official site to apply. The e-Visa costs $50 (payable via credit card); you'll also need to upload a recent color ID image along with your passport and ticket details.
Without an e-Visa, US passport holders must obtain pre-approval for a visa from the nearest Myanmar embassy or mission. Find out more at the Embassy of Myanmar's official site.
For more information, read this article: Myanmar Visa - Entry Requirements and How to Get Your Visa to Myanmar. More general information also available here: Myanmar Travel Information.
Visa Requirements for the Philippines
US passport holders may enter without a visa, for stays not exceeding 21 days. Visa Extensions can be obtained from Philippine Embassies, Philippine Consulates, or from the Bureau of Immigration within the Philippines. Read this article: Philippines Travel Information.
For more information on Philippines visas, proceed to the Philippine embassy's official site.
Visa Requirements for Singapore
US passport holders are not required to obtain a visa to visit Singapore; the Entry Pass permits a maximum stay 30 days. Read this article: Singapore Travel Information. For more information on Singapore visas, proceed to the Singapore embassy's official site.
Visa Requirements for Thailand
US passport holders can get a stamp on arrival, permitting them to stay for up to 30 days. A single 14-day extension may be obtained.
For general information about visiting Thailand (with visa info included), read this article: Thailand Travel Information. For visa-specific information, read this: Do I Need a Visa to Visit Thailand? For potential overstay issues, read about extending your Thailand visa.
Visa Requirements for Vietnam
US passport holders must obtain pre-approval for a visa from the nearest Vietnamese Embassy. 30-day or 90-day visitor’s visa may be obtained in the U.S. from either the Vietnamese embassy in Washington, DC if you're on the East Coast, or at the Vietnamese consulate in San Francisco if you're on the West Coast.
For a complete run-down of the ins and outs of securing a visa for your trip to Vietnam, read this article: Three Ways to Get Your Vietnam Visa.