People currently get antiretroviral therapy only when their white blood cell levels drop.
But a US-led study has now been cut short as early treatment was so beneficial for patients.
The United Nations Aids agency has called for everyone to get immediate access to the drugs.
Around 35 million people are living with HIV and more than 2 million start antiretroviral therapy each year.
The discovery of drugs to attack the virus has profoundly changed the way the disease is treated.
But there has been fierce debate about when treatment should start.
World Health Organization guidelines say treatment should start when there are fewer than 500 white blood cells in every cubic millimetre of blood.
The trial on 4,685 people in 35 countries, organised by the US National Institutes of Health, compared this approach with immediate treatment.
The trial started in 2011 and was due to run until the end of 2016.
But an interim analysis of the data showed that cases of Aids, deaths and complications, such as kidney or liver disease, had already been halved by early treatment.
All patients on the trial are now being offered antiretroviral drugs.
The director of NIH's National Institute of Allergy and Infectious Diseases, Anthony Fauci, said: "We now have clear-cut proof that it is of significantly greater health benefit to an HIV-infected person to start antiretroviral therapy sooner rather than later.
"Moreover, early therapy conveys a double benefit, not only improving the health of individuals but at the same time, by lowering their viral load, reducing the risk they will transmit HIV to others.
"These findings have global implications for the treatment of HIV."
Michel Sidibe, executive director of at UNAids, argued: "Every person living with HIV should have immediate access to life-saving antiretroviral therapy.
"Delaying access to HIV treatment under any pretext is denying the right to health."
Dr Steve Taylor, the lead HIV Consultant at the Birmingham Heartlands HIV Service, told the BBC the trial was hugely important.
"Not least that they will they will change the way HIV treatment is prescribed in the UK and around the world.
"Based on this study, people will be able to access treatment much earlier than currently, which is good for their own health and will reduce HIV transmission."
Deborah Gold, chief executive of the National Aids Trust, said: "These exciting results should dramatically change the approach to treatment for people living with HIV, both in the UK and internationally."
HIV drugs should be given at the moment of diagnosis, according to a major trial that could change the way millions of people are treated.