AUM at the end of September stood at £15.05bn (US$18.5bn), up from £14.54bn (US$18.4bn) at the beginning of the quarter.
Compared to a year earlier, when the asset management firm had £12.52bn of assets under management, AUM was 20% higher.
During the July to September period, the company experienced net inflows of £235mln, accounting for 46% of the increase in AUM.
"At the end of Impax's most recent financial year, I am pleased to report further significant growth for the company,” said Ian Simm, the chief executive of Impax.
“Notwithstanding a challenging backdrop for equity markets, Impax has continued to attract strong net inflows of capital and a healthy pipeline of potential new business from asset owners around the world that are seeking investment exposure to the transition to a more sustainable economy," he added.