Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula (CH3CH2)4Pb. It is a petro-fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that allowed engine compression to be raised substantially. This in turn caused increased vehicle performance and fuel economy. TEL had been identified chemically in the mid-19th century, but its antiknock effectiveness was discovered in 1921 by the General Motors research laboratory, which had spent several years attempting to find an additive that was both highly effective and inexpensive.

Concerns were later raised over the toxic effects of lead, especially on children. Lead also poisons catalytic converters and is a major cause of spark plug fouling. From the 1970s many countries began phasing out and eventually banning TEL in automotive fuel. In 2011, a study backed by the United Nations estimated that the removal of TEL had resulted in $2.4 trillion in annual benefits, and 1.2 million fewer premature deaths.

TEL is still used as an additive in some grades of aviation gasoline. Innospec has claimed to be the last firm legally making TEL, and as of 2013, TEL was being produced illegally by several companies in China. In July 2021, the sale of leaded gasoline for cars was completely phased out worldwide, prompting the United Nations Environment Programme (UNEP) to declare an "official end" of its use in cars on August 30 (although it is still available in aviation fuel for small airplanes).