The Mexican Institute of Social Security (Spanish: Instituto Mexicano del Seguro Social, IMSS) is a governmental organization that assists public health, pensions and social security in Mexico operating under the Secretariat of Health. It also forms an integral part of the Mexican healthcare system.

The IMSS was founded by Mexican President Manuel Ávila Camacho on January 19, 1943 to satisfy the legal precepts established in the Article 123 of the Mexican Constitution. It is constituted by representations of the workers, employers, and the federal government.

It is the largest social welfare institution in all Latin America.^{[citation needed]}

For some time, however, there have been festering signs of trouble in IMSS, such as serious financial problems that came to a head in early November 2010.

The Mexican Social Security law currently in effect, published in the Official Journal of the Federation (21 December 1995), is the legislative domain under which the IMSS carries out its operations.

Currently the law indicates that Social Security has the following purposes:

• Medical assistance
• Protection of basic necessities of subsistence
• Social services necessary for individual and collective well-being
• Giving out a pension which, depending on the completion of the legal prerequisites, will be guaranteed by the State

The law contemplates two domains, an "obligatory" one (funded by individual, employer and state contributions), and a "voluntary" one (aimed at workers in household industries and self-employed professionals).

The following items are excluded from the base quoted salary:

• Tools of trade such as tools and clothing
• Savings deposits, when they are made up of a weekly, biweekly or monthly deposit equally from the worker and the employer
• Additional voluntary contributions
• Contributions to INFONAVIT
• Food and lodging when they are given in an onerous manner
• Payments in coin or cash
• Rewards for attendance and punctuality
• Overtime, within limits established by law

• Flores Alvarado, A. and J.A. Moran Zenteno. The effects of the health care model of the IMSS-COPLAMAR program on the health status of the underprivileged rural population in Mexico. Mexico: Salud Pública de Mexico. 1989 (Nov-Dec 31(6):745-56.

• IMSS website