Fish Sauce vs. Soy Sauce: When to Use Each and Can You Substitute?

Fish Sauce vs. Soy Sauce: When to Use Each and Can You Substitute?

If you’ve ever stared at a recipe that calls for fish sauce and thought, “Can I just use soy sauce instead?” you’re not alone. I’ve made that swap in a pinch, and let’s just say... sometimes it works, sometimes it’s a total flavor disaster. So, let’s dig into what makes these two sauces tick, when you really need the real thing, and my honest kitchen-tested advice for swapping one for the other (or not).

What’s Actually in Fish Sauce and Soy Sauce?

Fish Sauce: Funky, Salty, Magical

Fish sauce is basically anchovies (or other small fish), salt, and time. That’s it. They ferment together for months, sometimes a year or more, until you get this salty, slightly sweet, umami bomb that smells a little intense but transforms everything it touches. The first time I opened a bottle, I gagged. Now I can’t live without the stuff. If you’re new to it, don’t let the smell scare you - a little goes a long way, and it mellows out in a dish.

Soy Sauce: Savory, Complex, Less Scary

Soy sauce is made from fermented soybeans, wheat, salt, and a starter mold. There are a million varieties (light, dark, Japanese, Chinese, low-sodium, etc.), but the bottom line is it’s also salty and umami-rich, but with more of a roasted, malty character. It’s less fishy (obviously), so people tend to reach for it more often - and that’s totally fine.

How They Taste Side by Side

This is where things get interesting. Fish sauce tastes like salty ocean funk (in a good way). It’s sharper and more assertive. Soy sauce is smoother, rounder, with a hint of sweetness and a definite “cooked” flavor. I once did a blind taste test for a stir-fry sauce with friends. Everyone could tell which batch had fish sauce: it was deeper, more savory, and just... punchier. But it was also polarizing! Some loved it, a couple hated it.

When to Use Fish Sauce

I use fish sauce for Southeast Asian dishes (think Thai, Vietnamese, Cambodian), especially anything with lime, chili, and fresh herbs. It’s the backbone of nuoc cham (Vietnamese dipping sauce), and it’s essential in pad Thai, larb, papaya salad, and even in some curries. I’ve tried skipping it in papaya salad once, and the whole salad tasted flat. No lime or chili could save it.

What surprised me: Fish sauce is killer in non-Asian dishes too. I sneak a dash into Caesar salad dressing (instead of anchovies), and it gives homemade Big Mac Sauce or even bolognese sauce a crazy umami boost. Just a few drops, though - I learned the hard way that more is NOT better.

When to Use Soy Sauce

Soy sauce is so versatile. I use it for Chinese stir-fries, Japanese dipping sauces, Korean marinades, and even in salad dressings or drizzled over rice. It’s a staple in my fridge. It’s also a great way to add saltiness and color to gravies or pan sauces (check out my pan sauce guide if you want to get fancy). I also use it in a quick Creamy Sriracha Mayo Sauce for a fast sushi bowl.

Fun fact: If you’re making a dish that’s mostly about the soy flavor (like a teriyaki or dipping sauce for dumplings), fish sauce is NOT the right swap. It’ll just taste weird.

Can You Substitute Fish Sauce for Soy Sauce (or Vice Versa)?

Short Answer: Sometimes, But Not Always

This is the part where I’ll save you some pain. If a recipe calls for just a teaspoon of fish sauce, and you don’t have any, you can use soy sauce plus a squeeze of lime juice or a splash of rice vinegar. It won’t be the same, but it’ll hit salty and tangy notes. Don’t do this for something like nuoc cham or pad Thai if you want it to taste authentic. I’ve tried, and it’s always disappointing.

Swapping soy sauce with fish sauce is riskier. Fish sauce is much saltier and more intense. If you try to use it 1:1, your dish will taste like the ocean and overwhelm everything else. If you’re desperate, dilute fish sauce with water (about half-and-half) and add a pinch of sugar to mimic some of soy’s sweetness. But honestly, just wait until you can get some soy sauce unless it’s a tiny amount in a complex dish.

What About Vegan Substitutes?

I’ve had luck with a good mushroom soy sauce or homemade umami broth (dried shiitakes, a bit of miso, soy sauce, and a splash of vinegar). There are vegan “fish” sauces out there, and some are actually decent, especially if you’re just using a dash in a noodle salad or as a seasoning. Don’t stress about using the “perfect” sauce for a vegan pad Thai - store-bought vegan alternatives are totally fine here.

How Much Should You Use?

This is one of my biggest sauce mistakes: adding too much fish sauce or soy sauce right at the start. Both are salty, and you can’t un-salt a sauce. I start with half a teaspoon of fish sauce to season a salad dressing for two people, then taste and add more if needed. For soy sauce, a tablespoon in a stir-fry for two is usually plenty. Always taste after mixing, especially if you’re using both. If using both in a recipe, use a light hand with each - you can always add more, but you’ll never be able to take it out once your stir-fry tastes like a salt lick. Been there, ruined that.

Texture, Color, and Cooking Tricks

Texture

Fish sauce is thin, almost watery. Soy sauce is a bit thicker, but still pours easily. Neither will thicken a sauce by itself, so if you want something glossy for a stir-fry, you’ll need a thickener (cornstarch slurry is my go-to - see my tips in this guide if yours always turns out runny).

Color

Soy sauce adds a deep brown color. Fish sauce won’t change the color of your dish much, which is great if you’re making something like a Vietnamese dipping sauce or want to keep a salad bright.

Cooking Tricks

I’ve found fish sauce can burn if you add it directly to a hot pan and let it sit. I once made a pad Thai where I dumped fish sauce onto a super hot wok and, wow, the smell nearly cleared out the house. Now I always add it with other liquids or off the heat. Soy sauce is more forgiving, but even then, if you let it reduce too much, it gets weirdly sweet and sticky (sometimes that’s good, but usually not what you want).

Tested Tips

• Start small: If you’re new to fish sauce, use half the amount the recipe calls for and taste. It’s way easier to add more than to fix a dish that tastes like low tide. If you overdo it, add a squeeze of lime and a pinch of sugar to balance.
• The add-it-late rule: Add fish sauce or soy sauce near the end of cooking, not at the start. If you boil fish sauce too long, the flavor gets harsh and the aroma can be overwhelming. Soy sauce can also lose its complexity if cooked too long. I learned this the hard way after simmering a noodle soup for an hour with both - it tasted flat and weirdly metallic.
• Salad dressing hack: For a killer salad dressing, mix 1 tsp fish sauce, 2 tsp lime juice, 1 tsp sugar, and 2 Tbsp water. If it tastes too salty, just add another splash of water. If it tastes too fishy, add more lime. I use this trick on shredded cabbage and carrot for a super-fast slaw.
• Rescue too-salty sauces: If you accidentally add too much soy or fish sauce, toss in a splash of water and a pinch of sugar. If it’s still too salty, add more veggies or cooked noodles to soak up the extra flavor. I’ve saved many a skillet this way.
• Store properly: Keep fish sauce in a cool, dark cupboard with the lid screwed on tight. It’ll last forever, but if the cap isn’t tight, your whole pantry will smell like a fish market (ask me how I know). Soy sauce is easier, but still keep it sealed so it doesn’t pick up weird fridge odors.
• Don’t stress on brands: There are fancy fish sauces, but honestly, most supermarket brands (like Three Crabs or Red Boat) are great for everyday cooking. Unless you’re making a dipping sauce where the flavor is front and center, don’t shell out for the ultra-premium bottles. Same for soy sauce - Kikkoman or Pearl River Bridge is just fine unless you’re really geeking out.

What Happens If You Skip Fish Sauce or Soy Sauce?

If the recipe only calls for a small amount, you can sometimes get away with skipping it. For example: I once forgot fish sauce in a Thai-inspired slaw, and no one noticed (especially after I added extra lime and chili). But if it’s a key ingredient, like in Aji Verde Sauce or classic nuoc cham, skipping fish sauce just doesn’t work - the sauce will taste flat and bland, no matter how much lime or sugar you add.

The same goes with soy sauce. If it’s the backbone of your stir-fry or dipping sauce, skipping it leaves you with veggies and blandness. In a pinch, try a little miso paste mixed with water and a splash of vinegar. It’s not the same, but it’ll get you closer.

Recipes Where Each Shines

• Fish sauce: Essential in Southeast Asian dipping sauces, Thai curries, and even in some fusion dressings. Try adding a dash to Creamy Jalapeño Cilantro Dip for a flavor twist.
• Soy sauce: Perfect for stir-fries, marinades, and salad dressings. I always add a splash to my Spicy Maple Bourbon Glaze for grilled meats, and it’s a must in homemade Copycat Shake Shack Sauce.

If you’re looking for more ways to pair sauces with different dishes, check out my guide to pairing sauces with sheet pan dinners and how to match sauces with grains. Both have a bunch of practical ideas and swaps.

Final Thoughts from My Kitchen

I’ve ruined more stir-fries and dressings than I care to admit by mindlessly swapping fish sauce and soy sauce. But I’ve also made some happy accidents and found new favorites. The real secret? Taste as you go. Start small. Don’t be afraid to experiment, but respect the fact that fish sauce and soy sauce are not twins. They’re more like cousins who show up to the same party with totally different vibes.

So next time you’re staring at that bottle of fish sauce or soy sauce, just remember: use what you have, but know what you’re aiming for. And if you mess it up? That’s half the fun. Happy cooking!