SEATTLE — Microsoft has made its most ambitious move in years to reassert itself in a technology market it once dominated.

The software giant said Monday morning that it would acquire LinkedIn in a $26.2 billion cash deal. The acquisition, by far the largest in Microsoft’s history, unites two companies in different businesses: one a big maker of software tools, the other the largest business-oriented social networking site, with more than 400 million members globally.

The deal is Microsoft’s biggest bet yet that the traditional software business is shifting quickly to cloud computing, a model in which customers rent software and other services delivered over the internet. While LinkedIn does not have the household name of Facebook, a much larger and more lucrative social network, it is the most widely used site for people to advertise their professional skills and work history.

It is also further evidence that Satya Nadella, Microsoft’s chief executive, sees the company’s future further and further removed from the PC software that once helped the company’s co-founder Bill Gates turn Microsoft into the world’s most valuable company.