Total net inflows were more than £1bn, driven mostly by clients in Europe and North America.
The completion of the acquisition of Pax World Funds, which took place in September 2017, added £2.9bn of assets.
Impax reported a profit before tax of £5.5m on revenue of £25.7m.
Ian Simm, chief executive of Impax, said: "I am pleased to report on a successful first half of the year. Alongside robust investment performance and net inflows of £1bn, we completed the acquisition of Pax World Management LLC , further extending our investment management capabilities and client base in North America."
Impax said assets under management have grown further since the end of the six-month period, to £11.8bn at the end of May 2018.
Shareholders will receive a dividend of 1.1p per share, plus a one-off special dividend of 2.6p per share. This represented a 57 per cent increase in the ordinary dividend.
During the six months the company announced the final closure of its third infrastructure fund, which raised €357m.