There are plenty of excellent dry mustard substitutes, many of which may already be in your pantry!
Dry Mustard Substitutes
Finding the best dry mustard substitutes isn’t particularly difficult, but the precise dry mustard flavor can be hard to duplicate. Dry mustard is a prepared condiment that is commonly used in curries, salad dressings and as a spice in pickled and fried foods. In the U.S., the dried mustard powder is made from yellow mustard seeds. Although it can offer depth and flavor to a recipe, dry mustard powder often crops up in such small amounts that it might not be worth going to the supermarket for. Fortunately, there are a number of great mustard substitutes such as fresh mustard, Dijon mustard, mustard seeds, horseradish powder, turmeric, and wasabi powder among others. 
Fresh mustard may be more perishable than dry mustard, but it packs a punch and works well as a substitute in practically all recipes. To get the most similar flavor to dry mustard, look for Dijon mustard. Otherwise, yellow mustard will do a pretty good job. You can substitute fresh mustard for dry mustard at a 1:1 ratio.
Mustard seeds are essentially one step away from being dry mustard powder! You can use a pestle and mortar or a coffee grinder to turn mustard seeds into powder. Being the mildest, yellow mustard seeds will create the most similar flavor to the American-style dry mustard. Brown seeds offer a medium level of spice, while black is the hottest – both are eaten more in Asia than the United States. Consider using less powder than advised if you are using ground brown or black mustard seeds. 
A spicier alternative to dry mustard, horseradish will also provide more texture to your recipe, which may or may not work. However, if you like a little more kick, want the texture and don’t have any dry mustard in stock, horseradish can be a good option. Begin by adding half the amount of horseradish as is recommended for dry mustard and if desired, increase the amount according to taste. 
A simple powder replacement, turmeric can provide an earthy flavor and similar color to dry mustard, but won’t bring the same spice to your dish. You might want to consider mixing this spice with horseradish or wasabi if spice is something you’re after. When using turmeric alone, you can replace dry mustard at a 1:1 ratio.
Another spicy option, wasabi will add heat and depth to your recipe in a similar but more intense way than dry mustard. Like horseradish, we recommend starting with half the amount and increasing accordingly based on personal preference.