The company, Swedish-owned but based in Luxembourg, made a loss in the second quarter of €1.9m ($3.02m; £1.5m), compared with a net profit of €1m in the year ago period.
Metro warns of "gloomy" conditions, "particularly the US, UK and southern Europe, and less so in other markets in northern Europe".
President/ceo Per Mikael Jensen said, as a consequence, the company's strategy would concentrate on developing the emerging markets of South America, Asia and Russia, where prospects were better thanks to rising media spending.
It is also seeking cost savings, including shedding 14% of its staff.
Data sourced from Guardian.co.uk; additional content by WARC staff