Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb(C₂H₅)₄. It is a 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 increased vehicle performance and fuel economy. TEL was first synthesised by German chemist Carl Jacob Löwig in 1853. American chemical engineer Thomas Midgley Jr., who was working for General Motors, was the first to discover its effectiveness as an antiknock agent in 1921, after spending 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. On cars not designed to operate on leaded gasoline, lead and lead oxides coat the catalyst in catalytic converters, rendering them ineffective, and can sometimes foul spark plugs. Starting in the 1970s, many countries began phasing out 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 fuel. Innospec has claimed to be the last firm legally making TEL but, 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, 2021.