The deal closed yesterday for an undisclosed amount, but Sky News places its value at £10m.
The seven-year-old firm, whose 1,000 customers include Dell, PepsiCo and British Airways, provides analytics software designed to help marketers see what people are saying about their brands in social media.
The deal will see Brandwatch integrate PeerIndex’s influencer analysis technology, which includes Twitter’s author database, with its own platform.
Brandwatch, which has 300 employees worldwide, said the deal will provide marketers and other users with greater insight and "smarter decision-making".
"Content without context only goes so far, and we’re telling the world just that with our acquisition of PeerIndex," said Brandwatch’s CEO Giles Palmer in a statement.
"Understanding more about the actual people behind the conversations happening online is crucial to marketers, and this is about creating technologies that will allow them to target audience segments at scale with astonishing accuracy."
PeerIndex’s technology and engineering team of 10 is expected to join Brandwatch’s development division, which will be led by PeerIndex’s CEO Azeem Azhar.
The deal comes after Brandwatch closed a series B round of funding worth S22m in May, bringing total funding to $31.7m.