The bank said its profits, which were at £315m for the six months to the end of September, would have been 22 per cent, were it not for the rand falling 8.2 per cent against the pound.
Currency movements also hit customer deposits, which fell 4.2 per cent to £21.7bn, but would have reported a 4.5 per cent increase on a currency neutral basis.
Banking was the best-performing division, with profits up 29.1 per cent to £229m.
Wealth and investment management profits were down 0.5 per cent to £37.9m, thanks to investment in the UK business, but the bank said these “growth initiatives… should support an increase in operating margin in the medium term”.
The asset management business was hit by “market volatility and lower performance fees in South Africa”, and reported an eight per cent decrease in profits, and funds under management fell to £70bn.
Earnings per share rose 13.3 per cent to 25.5p, from 22.5p.
Stephen Koseff, chief executive of Investec, said: “We are pleased with the progress shown by the group, with profits comfortably ahead of last year. Our investment to digitise and internationalise the wealth and investment operation will place it on a strong footing to continue its growth trajectory. We are well placed to continue our growth despite the rand weakness and market volatility.”
Investec also announced it is setting a new South African investment vehicle, Investec Equity Partners, to hold and build on the group’s portfolio of industrial companies.
The business will raise 10bn rand (£462m) and hold 45 per cent of the new unit.