At the end of June, the group's AUM stood at £18.1bn, up 25.8% from the end-March figure of £14.4bn.
"Despite recent market volatility, Impax has again delivered significant growth, with net inflows of over £960 million for the quarter to 30th June,” declared Ian Simm, the chief executive of Impax in the third-quarter update.
"With our assets under management at £18.1 billion, a record high, and an attractive pipeline of new business, it is clear that investor interest in the transition to a more sustainable economy continues to gather momentum. Notwithstanding the current Covid-19 crisis, interest from asset owners in Impax's investment capabilities remains very healthy," he added.
Shares in Impax were up 3.2% at 374p in the first hour of trading.