I-129 highlighted in red
|Auxiliary route of I‑29|
 (5.60 km)|
Nebraska: 3.21 mi (5.17 km)
Iowa: 0.27 mi (0.43 km)
|Existed||November 22, 1976 –present|
|West end||US 20 / US 75 / US 77 in South Sioux City, NE|
|US 77 in South Sioux City, NE|
|East end||I-29 / US 20 / US 75 in Sioux City, IA|
Interstate 129 (I-129) is an auxiliary Interstate Highway which connects South Sioux City to Interstate 29 in Sioux City, Iowa. Opened in 1976, I-129 is a 3.48-mile-long (5.60 km) route, running 3.21 miles (5.17 km) in Nebraska. At 0.27 miles (0.43 km), Interstate 129 is the shortest highway in the state of Iowa. All of the route's length is concurrent with US 20 and most of its length is concurrent with US 75. Interstate 129 is also one of only four auxiliary interstate highways to go into a state its parent route does not.
Interstate 129 begins along U.S. Route 20 on the western edge of South Sioux City, Nebraska, just west of exit 1, a cloverleaf interchange with U.S. Route 75 and U.S. Route 77. US 77 travels north through South Sioux City before ending at Interstate 29 in Sioux City while US 75 joins I-129 and US 20. One mile (1.6 km) later, I-129 / US 20 / US 75 intersect Dakota Avenue at a partial cloverleaf interchange.  U.S. Route 20 Business is designated along Dakota Avenue.
East of Dakota Avenue, I-129 / US 20 / US 75 travels south of South Sioux City and passes through rolling farmland. For the rest of I-129's length, the two directions of I-129 / US 20 / US 75 traffic are separated by a Jersey barrier instead of a grassy median. The three routes cross the Missouri River and immediately intersect Interstate 29 at an interchange. Due to the minimum amount of space along the Missouri River banks, the I-29 interchange is a modified two-level cloverstack interchange.  At I-29, US 20 and US 75 continue east around Sioux City and I-129 ends.
After the passage of the Federal-Aid Highway Act in 1968, the mileage which would eventually be manifested in over 1,400 miles (2,300 km) of interstate highway was allocated to the states. Iowa received the smallest allocation, one-half mile (800 m), for the southern bypass of Sioux City.  The proposed highway was planned to cost $22.5 million ($135 million in 2019 ), which included $15 million ($89.8 million in 2019) for the Missouri River bridge.  On the 1973 state highway map, the Iowa State Highway Commission showed the planned route on the state map for the first time.  Interstate 129 was opened on November 22, 1976. 
|State||County||Location||mi  ||km||Exit||Destinations||Notes|
|Nebraska||Dakota||South Sioux City||0.00||0.00||US 20 west – Jackson, Ponca||Western end of US 20 overlap; continuation past US 77 interchange|
|1||US 75 south / US 77 – Fair Grounds, Sioux City||Western end of US 75 overlap; signed as exits 1A (south) & 1B (north).|
US 20 Bus. east (Dakota Avenue north) / IBP Avenue south – South Sioux City, Dakota City
|Nebraska– Iowa state line|
I-29 / US 75 Business – Council Bluffs, Sioux Gateway Airport, Downtown Sioux City
|Eastern end of US 20 / US 75 overlap; signed as exits 1A (south) and 1B (north); I-29 exits 144A-B.|
|US 20 east / US 75 north – Fort Dodge, Le Mars||Continuation past I-29|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- Iowa Department of Transportation. The National System of Interstate Defense Highways: 1956–1996.
- "Nebraska Highway Reference Log Book" (PDF). Nebraska Department of Roads. 2015. p. 270. Retrieved January 8, 2017.
- Google (February 27, 2010). "Interstate 129" (Map). Google Maps. Google. Retrieved February 27, 2010.
- Federal Highway Administration. "FHWA By Day - December 13". Retrieved August 1, 2010.
- Federal Reserve Bank of Minneapolis Community Development Project. "Consumer Price Index (estimate) 1800–". Federal Reserve Bank of Minneapolis. Retrieved January 2, 2019.
- "Hot off the Wire". Carroll Daily Times Herald. Carroll, Iowa. June 2, 1972. p. 2. Retrieved August 9, 2014 – via Newspapers.com.
- Iowa State Highway Commission (1973). State of Iowa Transportation Map (PDF) (Map). Ames: Iowa State Highway Commission. Sioux City inset. Retrieved August 1, 2010.
- "2009 Volume of Traffic on the Primary Road System of Iowa" (PDF). Iowa Department of Transportation. January 1, 2009. Retrieved September 30, 2010.
|Browse numbered routes|
|← Iowa 128||IA||Iowa 130 →|
|← N-128||NE||N-133 →|