The Second Federal Republic of Mexico (Spanish: Segunda República Federal de México) is the name given to the second attempt to achieve a federalist government in Mexico.

Officially called the United Mexican States (Spanish: Estados Unidos Mexicanos), a federal republic is implemented again on August 22, 1846 when interim president José Mariano Salas issues a decree restoring the 1824 constitution.

Like the Mexican Empire, the First Federal Republic and the Centralist Republic it is a chaotic period, marked by political instability that results in several internal conflicts.

Mexico's loss of the war with the United States sees half the territory Mexico claimed become part of the United States.

Even though Antonio López de Santa Anna plays a major role in much of this history, he returns to the presidency yet again, selling northern territory coveted by the United States contiguous to territory it had just gained in the Treaty of Guadalupe Hidalgo.

The sale of the Mesilla Valley is for many the final straw, and liberals promulgate the Plan of Ayutla, calling for the overthrow of Santa Anna.

Santa Anna goes into exile and the liberals set about implementing their vision of Mexico.

Liberals enact a series of separate reforms, then the Constitution of 1857, collectively known as the Liberal Reform, which sparks a civil war, known as the War of the Reform.

The conservatives set up a parallel Mexican government and lose the War of the Reform.

After losing the war, conservatives seek another political alternative, which involves the second French intervention in Mexico, which, with Mexican conservative support, establishes the Second Mexican Empire.

Mexican conservatives' political interests are in tandem with the expansionism of Napoleon III of France.

Conservatives invite Maximilian Hapsburg to serve as monarch of the Second Mexican Empire.

Mexican republicans fight against the French invaders and are largely defeated on the battlefield, but Benito Juárez does not resign the presidency, and operates a government in exile, which the United States continuesto recognize as the legitimate Mexican government.

The republic is restored by Juárez in 1867 after the withdrawal of the French and the execution of Maximilian.

With conservatives discredited by their support of the ill-fated monarchy, Juárez is able to implement liberal policies.

This period of federalism in Mexico is widely known as the Restored Republic, lasting from 1867 to the 1876 coup of liberal army general, Porfirio Díaz, ushering in a long period of authoritarian rule, peace, and economic development known as the Porfiriato.

The liberal constitution remains nominally in force, with regular elections held that are increasingly seen as fraudulent.

The Constitution of 1857 is supplanted by the Mexican Constitution of 1917, as an outcome of the Mexican Revolution (1910–1920).