The ISO advocated replacing the capitalist system with socialism, a system in which society's collective wealth and resources would be democratically controlled to meet human need by those who produce that wealth, i.e. the working class. The organization believed that this working-class majority could end capitalism by leveraging their power over production through mass strikes.
Supporters of ISO referred to their beliefs as 'socialism from below', a term attributed to Hal Draper. This concept can also be traced back to the rules of the First International which stated: "the emancipation of the working classes must be conquered by the working classes themselves." ISO saw this as distinguishing themselves from socialists who work within the Democratic Party and from various forms of what they disparagingly term Stalinism — nominally socialist politics, usually associated with the former Soviet Bloc and the old Communist Parties. These are seen as advocating socialism "from above". Because capitalism is a global system, the ISO argued that capitalism could not be successfully overthrown in individual countries. They agreed with Leon Trotsky that socialism in one country is an impossibility. The ISO held that the former Soviet Union and Eastern Bloc were examples of bureaucratic, class-stratified states, not socialist societies; and that the People's Republic of China and post-revolutionary Cuba had emulated this model.
Some of the political theories adopted by the ISO had been developed in the British Socialist Workers Party (SWP), including that of "state capitalism" developed by Tony Cliff, the party's founder. State capitalist theory identifies the former Soviet Union and Eastern Bloc as exploitative class societies driven by military competition with private Western capitalism, rather than as the "deformed workers' states" that Trotsky maintained they were in The Revolution Betrayed. The organization tended to follow Cliff's view of these governments as state capitalist, although not all members held this analysis. After the split with the International Socialist Tendency in 2001, this particular characterization became less strict.
Following Vladimir Lenin, the organization believed the creation of a revolutionary workers' party was necessary in coordinating and building the power of a revolutionary working-class vanguard. However, ISO believed that the historical conditions in the United States were insufficient for the existence of such a vanguard party. For this reason, the organization saw itself as a preliminary group that could help to win reforms and raise consciousness until such time that a revolutionary party could be formed. Nonetheless, it aimed for a Leninist principle of democratic centralism in its internal deliberation process. The ISO emphasized the training of cadre, seasoned and educated militants. In theory, these cadre would build the organization as well as engaging in movement work, and would someday cooperate with other groups in order to build a new vanguard party.
The ISO supported struggles for economic, political, and social reforms while also maintaining that exploitation, oppression, war, and environmental destruction could not be eliminated until the overthrow of capitalism and its replacement with socialism.
The organization offered critical support to national liberation movements. Most notably, the organization advocated solidarity with Palestine and supported the Boycott, Divestment and Sanctions movement. ISO also supported Syrian revolutionary groups against Bashar al-Assad.
The organization advocated the right of gays and lesbians to marry as well as social validation of transgender identities. In the final years of its existence, the organization was more strongly aligned with socialist feminist ideas and particularly Black feminism and intersectionality.
Philosophically, the organization defended the orthodox Marxist tradition from postmodernism. ISO was somewhat open to Western Marxist and Marxist humanist thinkers.