Last week, Coursera, a rival Silicon Valley-based platform, announced 12 more universities were joining.
UC Berkeley will add two courses to the edX online offering this autumn.
The edX partnership is also promising to add further universities from "around the world".
Edinburgh University emerged last week as the first UK university to join in this current race to establish online university platforms.
The Scottish university joined the Coursera project, which has partners including Stanford and Princeton.
This year has seen major US universities pushing ahead with rival plans to make courses available for free on the internet.
It has been hailed as a first step towards a major shift in higher education - with implications for the current constraints on time, capacity and funding.
It raises the prospect of giving prestigious institutions a global reach and access to students around the world.
The edX project followed from a MIT prototype, called MITx, that launched with a single electronics course, entirely taught and assessed online.
The addition of UC Berkeley maintains the position of elite institutions offering a small number of courses customised for online delivery.
The two courses from UC Berkeley, free to users, will be in software and artificial intelligence.
MIT will offer courses in chemistry and computer science and Harvard will run courses in health statistics and computer science.
The promise to announce further international partners will raise speculation about whether any more leading universities in the UK are set to join.
There have been earlier pioneers in this online education field, but in recent months this has gathered momentum - and with such big university brands it is becoming much more mainstream.
Among the factors helping to push this growth have been advances in the technology, such as tablet computers and video on broadband, the expanding global demand for higher education and deepening financial pressures.
Online university courses have the potential to reach large numbers of people currently unable to access higher education.
It also allows individual institutions to globalise their courses and "scale up" their potential reach.
MIT has pointed out it would have taken 40 years to teach the number of students who successfully completed its prototype online course.
The rising cost of tuition fees, worries about student debt and stretched university budgets have also raised the idea of online courses providing a more affordable model.
The edX alliance is based on a not-for-profit principle - with the funding being drawn from Harvard and MIT.
Coursera is backed by investors and aims to generate revenue from its online visitors, rather than charging directly for services, following the pattern of Google.
Both of these university partnerships are making a distinction between a fully fledged campus-based degree and these online courses where certificates are awarded rather than a formal qualification.
This highlights two areas that will need to be resolved as the experiments develop - how students can be assessed for online qualifications and how they should be accredited.
The president of edX, Anant Agarwal, welcomed the addition of UC Berkeley.
"EdX is about revolutionising learning, and we have received a tremendous outpouring of excitement and interest from universities around the world.
"UC Berkeley is an extraordinary public institution known not only for its academic excellence but also for its innovativeness.
"With this collaboration, edX is now positioned to improve education more rapidly, both online and on-campus worldwide."
The emerging format war between online universities has accelerated, with the University of California, Berkeley signing up to Harvard and MIT's edX partnership.