Gap saw sales sink 5% to $1.14 billion (£647.4 million) and like-for-like sales fall back 8% during August.
Gap's international comparable sales, which includes the UK, faired slightly better falling just 2% for the month.
The biggest fall came from Banana Republic North America which saw comparable store sales slump 14% in August.
Sales for the 30 weeks to August 30 are 5% down on last year at $8.02bn (£4.5bn). Year-to-date like-for-like sales are down 10%.
Gap's chief financial officer Sabrina Simmons said: "Consistent with our strategy, we are pleased that we delivered merchandise margins above last year. Moving forward, each division remains focused on executing product and store strategies for fall."