Mark Suster, BuildOnline's chief executive, said it had targeted iScraper Germany because it was fast approaching profitability, had good staff and had important customers.
These include developer OFB Bauvermittlungs- und Gewerbebau, which has a £1.7bn project portfolio.
Suster said: "We are interested in the traction it has with customers. Iscraper had substantial contracts and long-standing relations with its German clients.
The acquisition gives BuildOnline a secure footing in the £131bn German construction market, where use of business-to-business internet systems is forecast to reach 34% this year.
In April, BuildOnline secured an additional £10m in funding and formed a strategic partnership with Balfour Beatty.