Austin ZIP Codes: The Full List, Two Counties, and Two That Belong to the IRS

All 74 ZIP codes USPS assigns to Austin

USPS lists 74 ZIP codes with "Austin, TX" as the primary place name, a figure this site's Texas ZIP code overview already cites in its per-city comparison table, putting Austin fifth among Texas cities behind Houston, El Paso, Dallas, and San Antonio. 72 of the 74 sit in Travis County. The other two, 78717 and 78729, sit in Williamson County to the northwest, covered in the section below.

Two Austin ZIP codes belong to the IRS, not a neighborhood

73301 and 73344 stand out from the rest of the list immediately: they don't start with 787 like every other Austin ZIP, and neither corresponds to a residential or commercial delivery area. Both are non-geographic ZIP codes USPS reserves for a single high-volume recipient: in this case, the IRS's Austin submission processing center, which handles tax return processing, ITIN applications, and taxpayer correspondence for a large multi-state region. 73301 is the better-known of the two; 73344 serves the same IRS campus, in the Walnut Creek Business Park area of Travis County, and carries no population of its own.

This site's Atlanta ZIP page covers the same pattern (one IRS-only ZIP, 39901), and Chicago carries 28 similar single-recipient codes for airports and banks. What makes Austin's pair distinctive is the numbering: 73301 and 73344 sit in the 733 block, the same three-digit prefix range used for northern Oklahoma ZIP codes, rather than Texas's own 787 block that every other Austin ZIP uses. USPS assigns non-geographic codes like these somewhat independently of the surrounding area's normal numbering sequence, which is how an IRS facility physically located in Austin ends up with a ZIP code that looks, on paper, like it belongs three states away.

The 787 prefix shared by Austin's other 72 ZIP codes is itself worth a note, because it's easy to conflate with something unrelated: 787 is also the area code covering Puerto Rico, in service since March 1, 1996, when it split off the old 809 code as explained on this site's country code page. A ZIP prefix and an area code are two completely separate USPS/NANPA numbering systems that happen to share three digits in this case. Austin's postal ZIPs and Puerto Rico's telephone area code have no relationship to each other beyond the coincidence.

Why two Austin ZIP codes are in Williamson County, not Travis

78717 and 78729 cover Austin's far northwest, an area that developed differently from the rest of the city. Starting in 1973, this section of what's now northwest Austin was organized as Williamson County Municipal Utility District No. 1, a special utility district set up to provide water and wastewater service to new subdivisions before the city itself had annexed the land. Home construction in the district began in 1975 and continued through the mid-1980s. The City of Austin didn't fully annex the area until December 31, 2008, under a Strategic Partnership Agreement first signed in 1998 and amended in 2004. The annexation itself converted the old MUD into the Anderson Mill Limited District, ratified by a vote of district residents in May 2009. By the time that happened, thousands of houses already existed under Williamson County's jurisdiction rather than the city's own utilities. The Milwood neighborhood nearby followed a similar path as part of the North Austin MUD annexation.

USPS ZIP boundaries don't automatically shift when a city annexes new territory, which is why 78717 and 78729 still carry an Austin mailing address today even though the underlying county line was set decades before the city limits caught up to it. It's the same category of mismatch this site documents in Atlanta, where ZIP and county lines don't match either, though the underlying cause differs: Atlanta's ZIP boundaries simply follow mail-delivery routes rather than county lines, while Austin's traces to utility-district development that predated annexation.