Gross new business inflows for the period totalled £23.4bn - well up on the first half of 2014.
But outflows amounted to £34.7bn, substantially higher than a year ago.
Aberdeen blamed weak investor sentiment towards emerging markets and "expected structural outflows" from some institutional clients.
Despite the net outflow of funds, revenue in the first half rose by 20% to £605.2m, while underlying pre-tax profit was 25% higher at £270.2m.
Assets under management stood at £330.6bn at the end of March - about £6bn more than the same period last year.
The company said it would pay an interim dividend of 7.5p per share, an increase of 11% on last year.
In early trading, Aberdeen's share price was down by nearly 2.9% to stand at 450.30.
Chief executive Martin Gilbert said: "I am pleased to report that the group has increased its underlying profits by 25% as we benefited from the diversifying effects of the acquisition of Scottish Widows Investment Partnership, which we completed a year ago.
"We remain strongly cash generative and we again increased our dividend, whilst also adding to our regulatory capital headroom.
"Gross new business inflows have continued to grow.
"However, they have been offset by outflows, which reflect changes in asset allocation driven by macro-economic factors and some structural outflows from certain clients."
He added: "Despite these headwinds we are well positioned for the long term: financially strong, with a global distribution platform and a diversified range of capabilities and solutions for the evolving investment environment."
The asset firm's results were released on the day that banking giant HSBC said it would make a decision on whether to move its headquarters out of the UK within months rather than years.
There has also been recent speculation that Standard Chartered - whose second biggest shareholder is Aberdeen Asset Management - may be considering a similar move.
Asked by the BBC News Channel if he had thought of moving his headquarters from Aberdeen, Mr Gilbert responded: "It is something a board should consider.
"But the reason HSBC and Standard Chartered are looking at it is the banking levy, which so far we haven't had imposed on us, so we're not in the same position they are."
Aberdeen Asset Management has reported a continued outflow of funds, after a net £11.3bn was withdrawn by investors in the six months to 31 March.