The £13M deal will see Micro Drainage merged with XP software, Cardno's own international water engineering software business.
The company's WinDes drainage design suite is currently used by every water service provider in the UK, and is widely used around Europe.
Cardno's Managing Director, Andrew Buckley, said: "By leveraging XP Software's extensive international network, Cardno has the ability to market Micro Drainage's product range worldwide as well as being to able to offer existing XP Software customers a complementary and in-demand product suite."
The software allows consultants to design storm-water drainage systems and create simulations, modelling the effects of flooding.
Cardno's XP software suite is already well-established in Australia and the US and, together with Micro Drainage's established market-base in Europe, they expect combined software sales to exceed 15,000 licences.
Micro Drainage's Managing Director, Marian Millerack, said: "In Cardno we have found an ideal partner to support our future growth and international expansion.
"Cardno has the unique perspective of knowing how to run a successful software company combined with an extensive team of in-house water resource practitioners who understand the importance of such software in designing sustainable and cost effective solutions for their clients," he added.