Thailand's most famous soup — hot, sour, fragrant, and the first thing most visitors try.
A clear or coconut-cream broth spiked with lemongrass, galangal, kaffir lime leaves, fish sauce, and lime — shrimp (kung) is the classic protein.
Tom Yum Goong (with shrimp) is the standard; Tom Kha Gai adds coconut milk and chicken for a creamier, milder version.
Every Thai restaurant has it; street stalls usually excellent; upscale versions have better prawns; quality gap between good and bad is enormous.
Tom Yum is meant to be spicy — if you want mild, say 'mai pet'; the spice and sour together is the point.
"Tom yum nam sai" (clear broth) is the sharper, more traditional version; "tom yum nam khon" adds evaporated milk or chilli paste for a richer, slightly creamier bowl — both are widely available.
Street-level bowls run 50–100 THB; upscale restaurant versions with larger prawns can reach 200–400 THB — quality and prawn size scale with price more than most other Thai dishes.