Interstate 238 (I-238) is a short auxiliary route of the Interstate Highway System in the San Francisco Bay Area, California, United States. Part of State Route 238 until 1983, and still considered the same route (part of Route 238) by the California Department of Transportation (Caltrans), it connects Interstate 580 in Castro Valley (where SR 238 continues south) with Interstate 880 in San Leandro. Despite running in an east–west direction, it is signed north–south, as the rest of Route 238 is more north–south than east–west.

The number does not follow established rules for numbering Interstates, as there is no Interstate 38. As it connects two auxiliary routes of Interstate 80, it would normally use a three-digit number ending in 80, but of the nine possible numbers, two (180 and 480) were in use by State Routes (the latter an Interstate until 1968), and the remainder were already in use by other California auxiliary routes. (I-880 was designated at the same time as I-238.)

This route is part of the California Freeway and Expressway System.

Although the 2.16-mile (3.48 km) I-238 goes in an east–west direction from Castro Valley to San Leandro, Caltrans officially signs it as a north–south freeway since the rest of State Route 238 is more north–south. The south (or east) terminus of I-238 is at its interchange with Interstate 580 and SR 238 in Castro Valley. From there, it enters into the southern portion of the census-designated place of Ashland, running parallel to its border with Cherryland. Then after entering San Leandro, I-238 terminates at Interstate 880.

I-238 and I-880 is used as an alternate truck route between Castro Valley and Oakland; trucks over 4.5 tons are prohibited through the latter on I-580.

The section of road that is now I-238 had no signed number before the 1964 renumbering; it was pre-1964 Legislative Route 228 (along with an unbuilt extension west to unbuilt State Route 61, which is still included in the Route 238 definition).

The segment from what is now I-580 to I-880 was built as a freeway in 1956. The rest of SR 238 south to Interstate 680 in Fremont was also planned to be upgraded to a freeway (parallel to present I-880), but after it was unsuccessfully submitted to the Interstate Highway System in October 1968, and after several lawsuits, it never got off the ground.

When present Interstate 880 was added to the Interstate Highway System as a renumbering of part of State Route 17, the short piece of SR 238 connecting I-880 to I-580 was also added; both were non-chargeable routes (not eligible for Interstate Completion funds). Both numbers â€” I-238 and I-880 - were approved by the American Association of State Highway and Transportation Officials (AASHTO) on July 7, 1983. The interchanges with I-580 and I-880 were rebuilt from 1988 to 1994, in part to add missing ramps between I-238 and I-880 towards the south. Prior to the completion of the ramps, access was provided by Hesperian Boulevard.

As the American Association of State Highway and Transportation Officials said on July 7, 1983, when approving the designation,

However, since the I-238 designation does not fit the overall national numbering sequence and was necessitated only because all three [-digit] combinations of I-80 have been used, the Committee has a further option to offer for your consideration. If the I-580 designation [were] continued from Castro Valley to San Lorenzo and then used in place of the proposed I-880 designation northerly to Oakland and over existing I-180 between Albany and San Rafael, then existing I-580 between Castro Valley and Oakland could be designated I-180. The Committee does recognize this option would involve considerable resigning, however.

Caltrans responded on July 27, 1983:

With the deletion of State Route 480 in 1991, the "480" designation is once again available. However, there has been no push since then to renumber I-238 to I-480.

In September 2006, a project began to reconstruct the entire length of I-238, including a reconfigured interchange with I-880 and an added travel lane in each direction. Additionally, almost all of the bridges and overpasses were replaced with new ones meeting current earthquake resistance standards. The project was completed in October 2009, 6 months ahead of schedule.

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, and do not necessarily reflect current mileage. The entire route is in Alameda County.