The nearest airports are in Chumphon, Ko Samui and Surat Thani. Ko Tao can then be reached by numerous ferry and high-speed catamaran services.
Nok Air sells a Bangkok to Ko Tao, air-sea combi ticket online. It includes a seat on one of their twice daily 86-seater turboprop services between Bangkok (Don Muang, DMK) and Chumphon Airport (CJM IATA), followed by a transfer to the ferry pier and a Lomprayah catamaran to Ko Tao.
If you have purchased a separate flight ticket, then various ferry operators have booths in the airport arrivals area selling ferry tickets which include a bus to the pier.
Lomprayah has an office on the main road in Hua Hin where you can catch its bus/ferry at 23:30 to travel to Ko Tao arriving at 08:45. The fare to Ko Tao is 1,250 baht (or 1,450 baht for a “VIP 24” bus as of May 2022). After Ko Tao, the ferry continues on to Ko Pha Ngan (1,350 baht) and then Ko Samui (1,450 baht).
The closest port to Bangkok for the island ferry service is Chumphon. Numerous agents sell tickets for a variety of boats of varying size and speed travelling in from the north. The fastest takes about 90 min, the slowest more than 5 hr.
There are two high-speed ferry operators from Chumphonː Lomprayah and Songserm. Their websites both have timetables and allow you to buy tickets online. They also both offer pickup services from the train and bus stations and from hotels. The fare for a Lomprayah ferry is 750 baht (as of May 2022) and can be as low as 550 baht if there is a special promotion running.
Another option is the night boats, generally leaving between 10pm and midnight, and offering bunks in large dormitories with fairly basic facilities. There are three operatorsː
All of these can be booked through the Salsa Hostel in Chumphon (+66 77 505005 or +66 89 4880321), who also arrange pickups.
A good solution if coming from Bangkok is to book "Sleeper Train #85"; leaving Bangkok at 19:30 and arriving in Chumphon at 04ː13 and then have a ticket booked for the 07:00 ferry. Both ferry operators can pick you up from the station where you can while away the connection gap of several hours. You can also buy your ferry ticket at the station when you get off the train. Lomprayah have a ticket office at the station and a ticket on a waiting bus is included in the price.
Catamarans are operated by Lomprayah and Seatran Discoverywith a travel time of around 3 hr. In addition, overnight ferries are a cheaper transport option. These depart around 23:00 and arrive the following morning at about 06:00 (subject to changes). As most of the boats are former cargo boats the passenger facilities are basic. The cheapest overnight ferries have large common unpartitioned sleeping areas and are cooled by fans only. Sleeping spaces are unallocated so try to arrive early. More expensive night ferries have a/c and partitioned single and double bunks. Your bunk will be assigned to you. Purchase tickets at the pier.
Bangrak Pier (aka "Big Buddha Pier") is your best bet if you are arriving at Ko Samui Airport (~10 minutes via taxi). Or are staying in Chaweng Beach area. Seatran Ferry has several trips to and from Ko Tao's main pier (Mae Haad). Seatran has large modern ferries, ~2 hours and 600 baht one way as of 2017.
Maenam Pier is on the Northern coast, ¾ hr to 1 hr from Ko Samui Airport. To confuse things, it is also known Lomprayah Ferry Pier. Lomprayah ferries cost 700 baht (or 600 baht with a special promotion) as of May 2022.
Nathon Pier is over on the Western coast of the island, about 90 minutes from the airport by car.
You might make the trip on diving charters from Ko Samui. Many have high-speed boats that can make the trip to Ko Tao in about 1 hr.
Lomprayah High Speed Catamaran or Seatran are the fastest and most comfortable way to get to Ko Tao. They run twice a day from Ko Samui, Ko Pha Ngan, Surat Thani, and Chumphon. They also have online booking, useful for checking availability at peak times and all their boats have air conditioning and movies for passengers.