The company said the fall in sterling since the EU referendum, had boosted exports, enabling it to cut prices, the BBC reports.
Asos, which targets people in their twenties, now has more than five million active customers in the UK.
Profits for the six months to the end of February rose by 14 per cent to £27.3m. Sales hit £889.2m, a rise of 31 per cent.
UK retail sales rose by 18 per cent and international sales leapt by 42 per cent.
“As a net exporter, sterling weakness has created a FX tailwind for the business which has enabled investment above previously planned levels into both price and proposition,” said chief executive, Nick Beighton.
As a result, Asos said it expected full-year retail sales to rise by between 30 and 35 per cent. Previously it said it expected sales to go up by 25-30 per cent.
However, its gross profit margin fell because of the price cutting, and its full-year profit before tax is predicted to be “broadly in line with market consensus”.
Its shares are down by nearly 6 per cent in early trade.
Asos now has a total of 14.1 million active customers worldwide, up from 10.9m in February of last year. It sells more than 85,000 products all over the world. As well as the UK site it also has local websites in the US, France, Germany, Spain, Italy, Australia and Russia.
In February its websites were visited 127 million times.