The 1990s (pronounced "nineteen-nineties", shortened to "the '90s" and often referred to as simply "the nineties") was a decade that began on January 1, 1990, and ended on December 31, 1999.

Known as the "post-Cold War decade", the 1990s are culturally imagined by many as the period from the Revolutions of 1989 until the war on terror of the 2000s. The dissolution of the Soviet Union marked the end of Russia's status as a superpower and the end of a multipolar world, and anti-Americanism grew. Russia was in economic and political chaos due to neoliberal shock therapy reforms under President Boris Yeltsin, while China was still recovering from a politically and economically turbulent decade, the 1980s. This allowed for the United States to emerge as the world's sole superpower during this decade.

The decade saw greater attention to multiculturalism as well as the advance of alternative media. In the built environment, the McMansion version of postmodern architecture remained strong throughout the decade. Public education about safe sex curbed the spread of the HIV virus in wealthy countries. Generation X young people often bonded over musical tastes. Humor in television and film was often marked by ironic self-reference mixed with popular culture references, in the manner of television series Buffy the Vampire Slayer. Alternative music movements like grunge, Eurodance, and hip-hop, became popular with young adults worldwide, aided by the rise in popularity of tiered pricing cable television, and the internet. New music genres such as drum and bass, post-rock, happy hardcore, denpa, and trance emerged in the 1990s. The computer game industry began to boom again, rivalling the music and movie markets. Neverwinter Nights was the first multiplayer online role-playing game to display graphics and ran from 1991 to 1997 on AOL. Video game popularity exploded due to the development of CD-ROM supported 3D computer graphics on platforms such as PlayStation, Nintendo 64, and PCs.

The 1990s saw advances in technology, with the World Wide Web, the evolution of the Pentium microprocessor in accordance with Moore's Law, rechargable lithium-ion batteries, the first gene therapy trial, and cloning all emerging and being improved upon throughout the decade. The Human Genome Project was formally launched in 1990. Building of the Large Hadron Collider commenced in 1998. In 1998, Nasdaq (established 1971) became the first stock market in the United States to trade online, using the slogan "the stock market for the next hundred years".

Environmentalism was divided between left-wing green politics, primary industry-sponsored environmentalist front organizations, and a more business-oriented approach to the regulation of carbon footprint of businesses. As more businesses were using information technology, the idea of the paperless office became a realistic long-term goal. The matter of persistent organic pollutants (POPs) was an ongoing concern. In 1995, the Governing Council of the United Nations Environment Programme (UNEP) called for global action to be taken on these "chemical substances that persist in the environment, bio-accumulate through the food web, and pose a risk of causing adverse effects to human health and the environment". The Intergovernmental Forum on Chemical Safety (IFCS) and the International Programme on Chemical Safety (IPCS) then prepared an assessment of the 12 worst chemical substances, leading to the Stockholm Convention on Persistent Organic Pollutants of 2001.

A combination of factors led to a realignment and consolidation of economic and political power across the world, such as the continued mass-mobilization of capital markets through neoliberalism, and the end of the Cold War caused by the dissolution of the Soviet Union. Network cultures were enhanced by the widespread proliferation of new media such as the internet, and a new ability to self-publish web pages and make positive connections on a wide variety of professional, political and hobby topics. The new medium was dubbed the information superhighway and soon used by the so-called 'tech savvy'. The digital divide was immediate, with access to the World Wide Web limited to those who could afford it and knew how to operate a home or business computer. The internet also provided a degree of anonymity for individuals skeptical of the government. Online anonymity and the semi-impersonal nature of communication by email and chat rooms led to the words troll and netiquette enter the English language. Online scammers and sex offenders scared many people off the idea of home internet, and traditional mass media continued to perform strongly. However, mainstream internet users were optimistic about its benefits particularly the future of e-commerce. Web portals, (such as AOL, Yahoo!, Yahoo! GeoCities and Lycos), a type of curated bookmark homepage, were as popular as searching via web crawlers. The use of country-specific web portals popularized the phrases netizen and global village.

Many countries were economically prosperous and spreading globalization during the 1990s. High-income countries experienced steady economic growth throughout the majority of the decade during the Great Moderation. Using a mobile phone in a public place was typical conspicuous consumption in the decade. In contrast, the GDP of the states of the former Soviet Union declined, as a result of neoliberal restructuring. International trade increased with the passage of the North American Free Trade Agreement (NAFTA) in 1994 and formation of the World Trade Organization (WTO) in 1995. The dot-com bubble of 1997–2000 brought great wealth to some entrepreneurs before its crash of the early-2000s. The economies of the Four Asian Tigers (Taiwan, South Korea, Singapore and Hong Kong), ASEAN, and Japan were hampered by the 1997 Asian financial crisis and the Lost Decades, causing the May 1998 riots in Indonesia and leading to the end of the 32-year Suharto dictatorship.

Major wars that began in the 1990s include the First and Second Congo Wars, the Rwandan Civil War and genocide, the Burundian Civil War, the Somali Civil War, the Algerian Civil War, and the Sierra Leone Civil War in Africa, and the Yugoslav Wars, the First and Second Chechen Wars, and the Tajikistani Civil War in the former Soviet Union, and the Gulf War and 1991 Iraqi uprisings. The Afghanistan conflict (1978–present) and Colombian conflict continued throughout the decade. The Oslo Accords at first seemed to herald an end to the Israeli-Palestinian conflict, but these hopes were in vain. However, in Northern Ireland, The Troubles came to a standstill in 1998 with the Good Friday Agreement, ending 30 years of violence.

During this decade, the world population grew from 5.3 to 6.1 billion people. There were approximately 1.35 billion births and 525 million deaths.