Tahini and tzatziki both show up in Mediterranean dishes and have historical connections to tarator sauce. You should have both in your kitchen if you plan to make Greek or Lebanese dishes, but they are not usually interchangeable. Here is a look at how they compare to each other.
How does tahini differ from tzatziki?
The most obvious difference between tzatziki and tahini is the fact that they are made from quite different ingredients. Tzatziki consists primarily of yogurt while tahini is vegan and made up mostly of ground sesame seeds with oil.
They differ in terms of their historical sources as well. Tzatziki’s origin is in India but there has been a strong Greek influence on its ingredients while tahini comes from the Middle East.
The area of flavor is where tzatziki and tahini differ most. Keep in mind that tzatziki is a complete sauce with multiple ingredients to enhance flavor and mouthfeel, while tahini is not usually used on its own but as an ingredient in other dishes.
Tzatziki 
Tahini
Tzatziki has a complex flavor profile that includes earthy and umami notes from garlic as well as tartness from lemon and herbaceousness from parsley or dill. Tahini’s flavor is simply nutty with a very mild bitterness. Tzatziki will also be thicker and creamier than tahini due to the yogurt and olive oil content.
Because it is made mostly from Greek yogurt, tzatziki is white and has an appearance similar to cottage cheese. There may be a green element from the cucumbers as well as the parsley or dill. Tahini has a smooth consistency and a light-brown color that makes it look a lot like creamy peanut butter.
If your recipe calls for one, can you use the other?
You can combine tahini with other ingredients to make a decent tzatziki substitute but it probably won’t work on its own if what you want is a sauce with tzatziki’s complexity and rich mouthfeel. Tahini’s flavor is too one-dimensional and its consistency is too smooth and thin.
Tzatziki can make a good substitute for tahini in some applications since it will contribute a more significant amount of flavor, but it might not be a good addition if you want the simplicity of plain tahini.
When should you use tahini and when should you use tzatziki?
Tahini is usually used as an ingredient in dipping sauces like tarator sauce, which combines tahini with lemon and olive oil among other ingredients. Tarator and tzatziki have a common ancestor in India’s raita sauce.
Tahini is also an essential ingredient in hummus where it adds a rich nutty flavor and helps to make the texture smoother. Tahini may also be used to make salad dressings and sauces for grilled meat. It is a common condiment for falafel. Because of its simple flavor profile, tahini is a versatile stand-in for dairy products in vegan dishes. You can even use it to make sweet treats like ice cream and smoothies.
Use tzatziki as a flavorful dipping sauce for vegetables and pita. It is also a traditional condiment for falafel, gyros, and kebabs. You can make a thinner version of the sauce by adding extra lemon juice or olive oil and using it as a salad dressing similar to ranch or caesar dressing. Like India’s raita sauce, you can use tzatziki to help soften the effects of spicy food. Serve it alongside wings or chili the way you would ranch dressing.