Ariba's platform focuses on business-to-business commerce transactions. It makes a natural fit with SAP's "broad customer base and deep business process expertise," SAP said in a statement.

The deal has been unanimously approved by Ariba's board and is expected to close in the third calendar quarter of this year, subject to stockholder and regulatory approvals.

When complete, the Ariba transaction will substantially grow SAP's footprint in cloud software, which got a previous boost from its recent $3.4 billion acquisition of SuccessFactors, a company focused on human resources applications.

At its Sapphire conference last week, SAP spelled out further details of its overall cloud computing strategy, with which it hopes to shoulder aside rivals such as Oracle as well as pure cloud vendors like Workday and NetSuite.

Ariba had $444 million in revenue during 2011 and has 2,600 employees, according to a statement. Its trading network is involved with more than $319 billion "in commerce transactions, collaborations, and intelligence among more than 730,000 companies," according to a statement.