The Foothill Freeway is a freeway in the Greater Los Angeles in the U.S. state of California, running from the Sylmar district of Los Angeles east to Redlands. The western segment is signed as Interstate 210 (I-210) from its western end at I-5 to SR 57 in Glendora, while the eastern segment is signed as State Route 210 (SR 210) to its eastern terminus at I-10. Under the California Streets and Highways Code, the entire Foothill Freeway is legally referred to as Route 210.

The Foothill Freeway name is a reference to Foothill Boulevard and the San Gabriel Mountains, both of which the freeway runs parallel to for most of its length. The freeway follows the foothills of these mountains, connecting the northeastern suburbs of Los Angeles with the Inland Empire. Historically, the Foothill Freeway spanned multiple numerical designations. Additionally, the I-210 designation has changed routings, previously including a portion of what is now the Orange Freeway (SR 57). East of Pasadena, the Foothill Freeway parallels, and in some parts replaced, the route of former U.S. Route 66.

The portion between I-5 and SR 259 in San Bernardino was up to Interstate Highway standards by 2007, but the eastern segment remains signed as a state route because the portion between SR 259 and I-10 had not met those standards. On February 26, 2020, construction in each direction took place to complete the standards required. The three-year project added lanes from Sterling Avenue in San Bernardino to San Bernardino Avenue in Redlands. Although construction was completed in September 2023, the eastern end currently remains "Route 210".

Route 210 is defined as follows in section 510, subdivision (a), of the California Streets and Highways Code:

Route 210 is from:
(1) Route 5 near Tunnel Station to Route 57 near the City of San Dimas via the vicinity of the City of San Fernando.
(2) Route 57 near the City of San Dimas to Route 10 in the City of Redlands via the vicinity of the City of Highland.

The section of Route 210 defined in paragraph (2) is not considered an Interstate Highway according to the Federal Highway Administration (FHWA)'s route logs.

I-210's western terminus is at its junction with I-5, near the Sylmar district of Los Angeles. From that point, the freeway's alignment is generally diagonal as it heads southeast through the northeastern San Fernando Valley and the Crescenta Valley. After leaving Los Angeles, it enters northern Glendale and then La Cañada Flintridge where it meets with the Glendale Freeway and Angeles Crest Highway portions of SR 2 before turning due south towards the junction with SR 134) in Pasadena. At this interchange, the Foothill Freeway shifts its alignment and direction, becoming an east–west freeway. From the north, the primary through lanes of I-210 become the unsigned northern stub of unfinished I-710, while from the east, the through lanes of the Ventura Freeway become I-210 as the Ventura Freeway reaches its official eastern terminus. After intersecting the northern terminus of I-605, I-210 then continues east to SR 57 in Glendora. Heading east from the SR 57 interchange until its eastern terminus at I-10 in Redlands, Route 210 is signed as a state route.

Portions of the Los Angeles County Metropolitan Transportation Authority's (Metro) light rail A Line runs in the median strip of I-210 from Pasadena to Arcadia, serving three stations at Lake, Allen, and Sierra Madre Villa.