Measure R was a ballot measure during the November 2008 elections in Los Angeles County, California, that proposed a half-cent sales taxes increase on each dollar of taxable sales (originating in or made from Los Angeles County) for thirty years in order to pay for transportation projects and improvements. The measure was approved by voters with 67.22% of the vote, just over the two-thirds majority required by the state of California to raise local taxes. The project was touted as a way to "improve the environment by getting more Angelenos out of their cars and into the region's growing subway, light rail, and bus services." It will result in the construction or expansion of a dozen rail lines in the county.

The ballot measure created an ordinance called the Traffic Relief and Rail Expansion Ordinance, which included an expenditure plan defining specific projects to be funded, timeframes for availability of funds, and expected levels of funding. The ordinance became effective on 2 January 2009 and is set to expire in year 2039. Projects to be funded include expansion of light rail and subway services, freeway expansions, and funds for local cities to spend on their own transportation infrastructure.

After the passage of Measure R in November 2008, the new sales tax rate in Los Angeles County rose to 8.75% (since 2013, 9-10%), second only to Alameda County in California (though there were a few cities whose sales tax rates exceeded the new rate). Before passage, the Los Angeles Economic Development Agency estimated that it would cost each county resident about $25 a year, and each family about $80.

The tax is expected to raise $40 billion over thirty years. After subtracting 1.5% for administrative costs, the remaining money must be spent as follows:

Examples of transportation projects and improvements cited by proponents of Measure R include beginning the so-called Subway to the Sea, get the Green Line light rail to Los Angeles International Airport, widen the 5 Freeway at the bottleneck before the Orange County line, and add carpool lanes. The sales tax froze regular fares until 2010 and froze fares for seniors, the disabled, students, and those on Medicare thru 14 September 2014.

Most of the projects depending on Measure R for money will require additional funding. This is because in order to secure political support, proponents aimed to offer "something for many constituencies", leading to an ambitious list of transportation projects, which relies in part on federal and state funds making up part of the funding of proposed projects, as has been the case for past projects. Planned expenditures included:

Among prominent politicians, Los Angeles Mayor Antonio Villaraigosa, county Supervisor Zev Yaroslavsky, and Assemblyman Mike Feuer (D-Los Angeles) were the most vocal proponents. The Los Angeles County Federation of Labor and the Los Angeles Area Chamber of Commerce supported it.

Having donated $900,000, the Los Angeles County Museum of Art (LACMA) was the "largest single donor to the effort to raise the sales tax in the county and build more mass transit". According to LACMA surveys, the top three reasons people do not visit the museum pertain to transportation: prospective visitors "live too far, there's too much traffic, and the museum is inconvenient to freeways and mass transit".