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Auxiliary Interstate Highways (also called three digit Interstate Highways) are a subset of highways within the United States' Interstate Highway System. The 323 auxiliary routes generally fall into three types: spur routes, which connect to or intersect the parent route at one end; bypasses, which connect to the parent route at both ends; and beltways, which form a circle that intersects the parent route at two locations. Some routes connect to the parent route at one end but to another route at the other end; some states treat these as spurs while others treat them as bypasses. Like the primary Interstate Highways, auxiliary highways meet Interstate Highway standards (with rare exceptions). The shorter auxiliary routes branch from primary routes; their numbers are based on the parent route's number. All of the supplement routes for Interstate 95 (I 95) are designated with a three digit number ending in "95": I x95. With some exceptions, spur routes are numbered with an odd hundreds digit (such as I 395), while bypasses and beltways are numbered with an even hundreds digit (such as I 695). Because longer Interstates may have many such supplemental routes, the numbers can repeat from state to state along their route, but they will not repeat within a state. There are three states that have no auxiliary Interstate Highways: Alaska, Arizona, and New Mexico. North Dakota has an auxiliary route, but it is unsigned, and Wyoming's does not meet Interstate Highway standards.

Auxiliary Interstates are divided into three types: spur, loop, and bypass routes. The first digit of the three digits usually signifies whether a route is a bypass, spur, or beltway. The last two digits are derived from the main Interstate Highway. For instance, I 515 contains an odd number in the first digit (5), which indicates that this freeway is a spur. The last two digits signify the highway's origin. In this case, the "15" in I 515 shows that it is a supplement to I 15. Exceptions to the standard numbering guidelines exist for a number of reasons. In some cases, original routes were changed, extended, or abandoned, leaving discrepancies in the system. In other cases, it may not be possible to use the proper number because the limited set of available numbers has been exhausted, causing a "non standard" number to be used.

A bypass route may traverse around a city, or may run through it with the mainline bypassing. In a typical 3 digit Interstate Highway, bypasses usually have both its two termini junctioned with another Interstate highway. Bypass routes are preceded by an even number in the first digit.