Skytrain (BTS)
This is the easiest way to get to Siam Square. Siam Square is the most central neighborhood of Bangkok and can be reached by both the Sukhumvit Line and the Silom Line of the BTS Skytrain system. The easiest entry point is Siam station, right on top of Siam Square and in the middle of the entire Siam area. It acts as the exchange station between both lines, so expect a lot of commuters during rush hour. The Sukhumvit Line continues east and passes Chit Lom station, which is close to Ratchaprasong, and Ploen Chit which serves the road of the same name. One stop west of Siam on the Silom Line is National Stadium, a handy stop that's close to MBK, Jim Thompson's House and the cheap guesthouses at Soi Kasemsan 1.
The Sukhumvit Line and the Silom Line provide direct connections with the districts of the same name. There is also a direct connection with Phahonyotin and the Chatuchak Weekend Market if you take the Sukhimvit Line to the north. If you're coming from Rattanakosin or Khao San Road, you'll first need to take the Chao Phraya Express Boat to Sathon (Taksin) pier. From there, continue your way along the Silom Line to Siam station. The whole ride takes around 40 minutes (but is easier than trying to find the right bus!)
By Boat
If you're staying at Khao San Road, the Saen Saep Express Boat is your best bet to get to Siam Square. It is only a short walk to the Golden Mount in Rattanakosin, where you can get on the boat. The service operates every day from 5:30AM to 8:30PM. A single trip from the Golden Mount takes about 25 minutes and costs around 8 to 20 baht. The Golden Mount Line brings you to Saphan Hua Chang pier for Siam Square, and terminates at Pratu Nam pier (for Ratchaprasong intersection). You can switch to the NIDA Line at Pratu Nam pier, which brings you to Chit Lom pier, Witthayu pier and goes on to Sukhumvit and Ramkhamhaeng.
By Bus
Siam Square is among the most congested districts of Bangkok. Combine that with the complexity of the bus system, and it's obvious that you're better off using other ways of transport. However, if you're coming from or going to the Grand Palace area in Rattanakosin, the bus is the only way to get a direct connection. Bus 15 goes from Ratchaprasong intersection, Siam Square and MBK Center to Hualamphong Station, Yaowarat Road, Charoen Krung Road on to Tha Chang pier (next to the Grand Palace). An alternative is bus 47, which starts at Rama IV Road in Silom, goes through Phaya Thai Road, passes MBK Center onto Luang Road, Worachak Road, Lan Luang Road, Ratchadamnoen Klang Road (for Khao San Road and ends at Na Phra Lan Road (at the Grand Palace in Rattanakosin).
If you're coming from Suvarnabhumi International Airport, the quickest way is to take bus AE3 (signed as going to Sukhumvit). It costs 150 baht and runs along the Bang Na-Trat Expressway and Sukhumvit Road to Ploenchit Road and Ratchaprasong intersection. It then loops around and heads back out to the airport. You can also take bus 552, which goes to Skytrain station On Nut in Sukhumvit. From there, you can continue your way with the Skytrain. It's a long ride though, and depending on traffic it could take between 1 to 2 hours.
By Train
Hualamphong Train Station, while quite a walk from Siam Square itself, is covered as a part of Siam Square in this article. The easiest way to reach it is by taking the Skytrain from Siam station to Sala Daeng station. There, switch to the MRT metro system at Si Lom station and get off at Hua Lamphong station. From there, all of Thailand is only one train ride away.
