An ersatz good (UK: /ˈɛərzæts/ AIR-zats, US: /ˈɛərzɑːts/ AIR-zahts, both also /ˈɜːr-/ UR-) is a substitute good, especially one that is considered inferior to the good it replaces. It has particular connotations of wartime usage.

Ersatz (German: [ɛʁˈzats] ^ⓘ) is a German word meaning substitute or replacement. Although it is used as an adjective in English, it is a noun in German. In German orthography noun phrases formed are usually represented as a single word, forming compound nouns such as Ersatzteile ("spare parts") or Ersatzspieler ("substitute player"). While ersatz in English generally means that the substitution is of unsatisfactory or inferior quality compared with the "real thing", in German, there is no such implication: e.g., Ersatzteile 'spare parts' is a technical expression without any implication about quality, Kaffeeersatz 'coffee substitute' is a drink from something other than coffee beans, and Ersatzzug 'replacement train' performs a comparable service. The term for inferior substitute in German would be Surrogat, which is cognate to the English word "surrogate".

In the opening months of World War I, replacement troops for battle-depleted German infantry units were drawn from lesser-trained Ersatz Corps, which were less effective than the troops they replaced.

Another example of the word's usage in Germany exists in the German naval construction programs at the beginning of the 20th century. In this context, the phrasing "Ersatz (ship name)" indicates that a new, larger or more capable ship was a replacement for an ageing or lost vessel. Because German practice was not to reveal the name of a new ship until its launch, this meant that the ship was known by its "Ersatz (ship name)" throughout its construction. At the end of World War I, the last three ships of the planned Mackensen class of battlecruisers were redesigned and initially known simply as the Ersatz Yorck class, since the first ship was considered to be a replacement for the lost armored cruiser Yorck.

The Allied naval blockade of Germany limited maritime commerce with Germany, forcing Germany and Austria-Hungary to develop substitutes for products such as chemical compounds and provisions. More than 11,000 ersatz products were sold in Germany during the war. Patents for ersatz products were granted for 6000 varieties of beer, wine and lemonade; 1000 kinds of soup cubes; 837 types of sausage and 511 assortments of coffee.

Ersatz products developed in Germany and Austria-Hungary during this time included:

Germany also stretched its supply of petrol with 'gasohol' (Benzolspiritus), which by today's standards would be classed as E25 petrol, consisting of 75% petrol and 25% distilled alcohol, likely ethanol.^{[citation needed]}

In World War II, Ersatzbrot (substitute bread) made of potato starch, frequently stretched with extenders such as sawdust, was furnished to soldiers as Kommissbrot, a dark German bread baked from rye and other flours used for military rations, and also to prisoners of war. One recipe reportedly discovered in the "Food Providing Ministry" (Reichsnährstand) in Berlin, labeled "(Top Secret) Berlin 24.X1 1941", contained 50% bruised rye grain, 20% sliced sugar beets, 20% "tree flour" (sawdust), and 10% minced leaves and straw.