Phuket and Koh Samui are Thailand's two best-known island destinations, and first-time visitors to Thailand often weigh one against the other.
Size and variety
Phuket is considerably larger and more developed, with a wider range of beach towns, price points and activities packed into one island — Koh Samui is smaller and generally quieter, with fewer distinct "areas" to choose between.
Getting there
Phuket has a major international airport with direct long-haul flights; Koh Samui's airport handles mostly domestic and regional connections, meaning many travellers reach Samui via a connecting flight through Bangkok.
Nightlife and pace
Phuket's Patong has a considerably bigger, louder nightlife scene than anything on Samui — if that's not what you want, both islands have quieter areas, but Samui as a whole leans calmer overall.
Which to pick
Phuket suits travellers wanting variety, more direct international flight options, and a wider range of day trips (Phi Phi, Similan, Phang Nga Bay all sit within reach). Samui suits those prioritising a smaller, more laid-back island with easy access to Koh Phangan and Koh Tao nearby.
Food and cost
Both islands have a similarly wide range of price points from street food to fine dining — cost differences come down more to which specific area and hotel tier you choose than to the island itself.
If you can't decide
Some travellers combine both on a single Thailand trip via a short domestic flight between them — worth considering if your total trip length allows for two island stops rather than forcing a single choice.