About a year ago, I was complaining about the lack of good Thai restaurants in Paris, despite a huge number of mediocre Chinese/Thai places, to an antiques dealer who has spent significant time in Thailand and seems to be knowledgeable about food. He recommended Thabthim Siam, which he said has authentic and really good Thai food.
This week I finally went (twice) to this simple restaurant, which claims to serve "cuisine Thai traditionelle." I can't vouch for the the authenticity, but the food is good. My first meal consisted of two daily specials -- shrimp and vegetable nems made with battered (rather than wrapped) whole shrimp and shellfish with red curry -- and an indescribable but excellent dessert of curls of jackfruit stuffed with coconut-infused sticky rice. Feeling that I should try some normal menu items, on my return visit I had shrimp and lemongrass soup (tam yam koun), chicken with green curry sauce, and an assortment of sorbets (mango, coconut and passionfruit). The flavors in all my dishes were beautifully blended, spicy but not hot, leaving me happily warmed inside.
Thabthim Siam is in a little section of Paris where no tourists go, even though it is not far from the beaten path. The nearest Métro station is Liège, where I don't think I have ever before gotten off, but which is located in the 9ème between Gare St-Lazare and Place de Clichy. Really not inconvenient at all.
Thabthim Siam, 28 rue de Moscou, Paris 8ème, Métro Liège or Europe.